Author's Note: Skipping the Section on Culture, Hive minds and all that.
Apparently hive minds are common, who knew? And after running into four of them they all decided to team up in the interest of playing to their collective strengths. So with that in mind, the Chimera Gestalt. Garrus Vakarian
Biology
The Chimera Gestalt gets its name from the first of the so-called Hybrid fleets, itself getting the name from a creature in Human mythology, something about a critter with four heads each from a different animal? Apparently it's sort of like a blanket term for an artificially created hybrid for them. Appropriately enough the Chimera Gestalt also has four heads, or rather representatives of their respective hive minds within the Gestalt each of which has their own mixed bag of history with the rest of the universe, the specifics of their policies are better covered in the history section below for now let's go over biology. Firstly the one we're all the most familiar with.
Rachni-A race of hyper resilient insect hive mind that were built to thrive in extreme environments, and normal environments, and pretty much any sort of environment really. Their brains are wired differently where they see sound and hear colour and this made talking to them pretty difficult, unless you're a Hanar apparently. Aside from that they're typified as four legged and stand up right with two tentacles for arms that apparently are fully functional as grasping appendages and for spraying acid, which is typically used to dissolve material to build their hives with but also make for deadly battlefield weapons. I haven't been able to get much more information on their anatomy from first hand experience, the Krogan I talked to about it said that they taste delicious boiled.
Thorian-Not so much a race as a fungus. A hyper adaptable fungus with a predisposition towards symbiosis and parasitism with psionics on the side. This stuff eats raw genetic material, swallowing the Nucleis and other Organelles whole, and so long as it has enough biological matter to do so uses that Nucleus as a blueprint to build a whole new organism, apparently for the purpose of spreading itself around and has thus learned how to reproduce all of the native wild life and plants of the planet we found it on of Feros. The only organisms they can't seem to be to do this to are DAP based ones like ourselves and the Quarians. Don't ask me how it does this, you're lucky I know what an Organelle is. The Fungus can survive in any environment but only thrives on garden worlds, and serves as the back bone of the Gestalt able to mass produce any of it's constituent creatures literally growing them out of the ground. It's favourite though seems to lean towards Thresher Maws, partly because they help to break up and till soil for growing new organisms, but mostly for the sheer raw size and power that they offer.
Prethoryn-A race of nomadic, titanic Extra-galactic organisms with a predisposition towards eating planets. These things apparently came out of the far end of Union territory running away from Reapers and attempted to eat the Galaxy as part of their preparations to flee to another. I understand wanting to run away from the Reapers, but I'm not willing to become a snack to do it. These things tend to reproduce by suiciding themselves on a life sustaining world, gushing out their bodily fluids into a sea of acid which is then used to break down all the organic material on or below the surface of the planet which is then condensed into mountain sized boil like 'Wombs' that eventually burst to give birth to new Prethoryn. Lately they've developed an appetite towards Eezo and the Biotic abilities they grant which combined with their pre-existing psionic abilities they use to terrifying effectiveness to wreak havoc into enemy ships. Surprisingly enough these things don't actually eat organic matter to sustain themselves, only puke all over it and mold it into more of their kind. No, what they eat is something called Vacuum energy. According to the Union scientist I talked to Vacuum energy, also known as Zero Point Energy is the sort of background noise of the entire universe, without which things get weird. Their words, not mine, Zero Point energy sort of prevents the creation of something called exotic particles which in very small amounts can do things like open up worm holes. Lovely to think about what kind of damage they do in large amounts. The larger 'Manta Ray' like organisms serve as carriers and share a symbiotic relationship with the smaller 'Bat' Like Shriekers who act as fighter craft for the Prethoryn.
Baol-Never saw the Baol in person, so I'm mostly gonna be relaying second hand information here. An ancient Plantoid type organism from the era of the last cycle, they are typified by a three stage life cycle. Stage 1 seed, normal harmless little seeds. Stage Two Sprout, which seems to have been the stage the one the Union thawed out was stuck on. In this stage the sprout acquires arms and legs and begins walking away from the tree that dropped them, stopping every so often to re-root to absorb nutrients and water while staying in a prolonged germination period. It's only after taking sufficient distance and enough time has past that this thing takes root properly and grows into another massive 'Tree of Life' similar in size to the one that dropped it grows. Apparently these huge things act as sort of psychic transmitters between worlds, which help to connect Chimera's expanding hive mind across multiple worlds.
History
The Gestalt is not one unified force but rather four smaller individual forces, each with their own unique histories with the Galactic community that has only recently come together. As such we will go over the individual histories of these four creatures.
Rachni-The one the council is most familiar with, unfortunately. The Rachni have a long and unfortunate history of being looked at by other races, being seen only for their potential for warfare and being exploited for it. The Protheans did this, the Reapers did this, we almost did it. From this it's entirely understandable where the Rachni's hostility came from that led to the Rachni wars, where their chief strategy at the time consisted of camping out on worlds too harsh for the rest of us to step foot on where they bred like crazy and built ships of war to attack other planets and pillage them for resources. Then the Salarian's discovered the Krogan, uplifted them, and then they had the means to fight the Rachni on somewhat even ground storming their nests and stomping out their queens after three centuries worth of conflict. Outside of these periods of bloody history the Rachni actually seem to be quite peaceful, I confess to having a hard time reconciling the stories of blood thirsty monsters with the Rachni queen gently cooing to her young.
Thorian-The Thorian is pretty recent as far as additions to the galactic community goes, it allegedly had a foot hold in the age of old before the Protheans fire bombed it's previous consciousness to smithereens. After that it nestled itself into a Prothean colony on what is currently Feros eating any Prothean corpses that it could get its slimy... tendrils? Roots? Doesn't matter, moving on. It ate and enslaved a bunch of Protheans and acquired their ambitious militaristic streak and now has a god complex. In more recent years it was dug up by Sovereign when it couldn't crack the Prothean data-cache and acquired a sort of psychic cipher from it, then Sovereign tried to have the damned thing killed when it figured out that the Thorian can reverse and immunise people against indoctrination. Currently the Thorian is being sprinkled throughout the galactic community across various agri-worlds in order to have it copy the crops and then feed it to the masses in order to mass immunise the entire galaxy.
Prethoryn-The Prethoryn appeared in the galaxy on the tail end of another battle with the Reapers, the damned squid bastards left them so scared witless that they collectively decided that the only hope for survival that they had was to flee as fast and as far as possible, and in order to replenish their numbers lost fighting against the Reapers attempted to eat all life in the galaxy. At this point they ran into the Union who largely succeeded in quarantining them and their suicidal charges into garden worlds to a specific region of space, apparently as part of a larger plan to throw them at the Reapers down the line. With Sovereign's appearance they finally started losing their patience and so almost resorted to drastic measures when one Asari who shall remain nameless basically walked up to a couple of them and tamed them, so now the shrieking monsters are on our side. Yeh. Currently the remainders of the Hybrid fleet are getting a shiny new coat of Dragon Scales to increase their durability, courtesy of the Thorian.
Baol-Another left over of the age of old, the Baol were a race of telepathic plant people. They were threatened by the Zha'til, discovered by the Protheans who took one look at their ability to photosynthesise massive quantities of light into psionic energy, transmit psychic messages across star systems that the Zha'til couldn't hack and basically concluded 'We can work with this' and so they took the Baol and brought them to every corner of the Galaxy. Then the Reapers came along and cut down every 'Tree of Life' in the galaxy the psychic backlash driving the Baol into despair and madness. The only reason any of them were able to survive is because the Prothean's put a bunch into cryogenic storage in order to restore their telepathic communication network after the Reapers were gone. Then the Zroni blew up the Galaxy and the Baol were lost in the ensuing chaotic tides of broken reality.
More recently Hydra invaded Citadel space, and in the ensuing scramble a single Asari was able to tame the Chimera Hive Fleet through classified methods and broker a deal between the four hive consciousnesses in order to share their collective resources. In the Gestalt consciousness that the four creatures share we have 'The Mother' (Rachni) 'The Father' (Baol) 'The King' (Thorian) 'The Craven' (Prethoryn) here we have a balanced mixture of compassion, wisdom, ambition and fear to govern the Gestalt's decision making processes, hopefully they don't end up in a tie or else we could be in trouble. In any case, moving on.
Naval Forces
The Union has broadly divided the Prethoryn into several Hive fleets, since until the Chimera Gestalt they lacked the means of long range intrasystem communication each of the hybrid Hive Fleets are governed by a Queen type organism. The Hybrid Fleets as of the time of writing this report are:
Hydra: Originally the first Hive Fleet to enter the Citadel system. Normally you'd need a hyper drive to pull it off but apparently after eating a mountain of Eezo from a dumping planet the Hive Fleet gained the ability to dramatically reduce their mass allowing their worm hole jumping capabilities to sky rocket. After that they suicided themselves on Virmire and started spreading out to Terminus and Citadel space. Hydra apparently refers to a big multi-headed serpent that only grows more heads the more that you fight it from human Mythology. Currently on its way back into Union space in order to help bring the other Hive Fleets into the fold.
Chimera: The Hive Fleet that split off from Hydra and entered the Terminus system, at which point it then met the other three hive gestalts and formed a new alliance first with each other to form a single governing body, and then with the Citadel and Union in the name of mutually assured survival. Currently on its way through Union space in order to either bring the rogue Hive Fleets to heel.
Tyran: A Hive fleet placed Under the command General Victus, originally the lot that invaded citadel territory, luckily the area the shrieking monsters chose to invade has been long contested territory with Terminus warlords and only rudimentary plans to colonise the system had been drawn up. After getting kitted with all the bells and whistles by Chimera it's currently on it's way to Gellix where it will use the planet as a spawning pit to begin expanding on it's naval power.
Larger and more numerous than the Hybrid hive fleets, we have the Prethoryn Hive fleets, each of them shaped and tempered by conflict with the Union and it's forces.
Echidna: The first of the Hive Fleets, Characterised by sheer numbers, apparently. The Dragoon's consider them kind of generic meaning that aside from the acid and psionics they don't have a lot of tricks up their sleeve, only numbers.
Gorgon: These ones ate a few Lithoid worlds, which apparently didn't give them much in the way of bio-mass, cause Lithoids, but did embed pieces of Lithoid stone into their overall skeletal structure similar to one of the procedures to enhance the Dragoons, in short, they eat rocks and grow, piss and excrete a Rare Crystal point defence grid, Exotic gas/plasma and Mot mines. Good luck sneaking up on these ones.
Dracula: Spent enough time watching Liches reanimate their dead and throw them back at them that the bastard's learned the same trick, when dealing with these things it's complete annihilation of the body, if you don't want the dead mutinying against you on the ship you're fighting with it's either out the airlock or into the incinerator with the bodies, not to mention complete annihilation of Prethoryn Corpses.
Surtr: Fire breathing monsters. How do they breathe fire in space? Ask the Draconian's they're the one that inadvertently showed them. According to Shepard you just use telekinesis to vibrate whatever you want hot and smoking and boom, fire. Now imagine that happening to entire ships.
Currently these four hive fleets are engaging with Hydra and Chimera, the plan, as it is, is to drag these bastards off to the Lithoids so-called slag worlds where they have already infested with Thorian Spores, crash them into the planet's surface and recycle the poor schmucks into more allied fleets. So yeh, more attack dogs to eat Reapers, Fantastic I guess?