Mikey was the first. Everyone thought he'd found some critter out the sewer, having gotten trapped down here. Usually only rats and bugs crawled around in the sewers, but occasionally something else got down in by accident. No one expected it be so much more than what it initially appeared to be, a critter that Mikey had found and adopted. But when asked Mikey said he hadn't found it. The squirrel had just appeared as he'd been playing around. Shrugging it off as Mikey simply having been too distracted to see it until it suddenly was in front of him, they thought nothing more of it and Splinter allowed him to keep the pet.
It became apparent quickly that something was not entirely right though. For starters ,the squirrel did not try to escape and was oddly attached to Mikey. When his brothers reached out to pet the new addition to the family, Mikey pulled away and even warned them to keep their distance. It was so unlike the kind and boisterous young turtle that it startled everyone. Mikey shared almost everything he had with his brothers, anything to bring a smile to their face and keep his family happy. For Mikey to claim something as his and his alone and no one could even touch it, it made clear that there was something strange going on.
In favor of keeping the peace until he figured out what drove his youngest, Splinter let it slide and warned his other sons to leave Michelangelo's pet alone for the time being.
Raphael was next. Unlike with Mikey, it wasn't as obvious with Raph. Not the sharing kind and a bit of a loner, there were hardly any incidences with his pet. Splinter wondered tohugh where he possibly could have found the little black creature, but his son much like Michelangelo could not give him a clear answer. "It was just sitting there."
Splinter's keen senses quickly deduced that Raphael's pet, too, was curious to say the least. His protectiveness of the creature was apparent to the master ninja, but went unnoticed by his other sons whom simply did not even try to reach for the black thing. They knew not to mess with Raphael. No Splinter became aware how very in tune the animal was to Raphael. It strangely mimicked the young turtles' mood swings, snarling when Raphael stomped away in anger or making high-pitched squeakes when he was teasing his brothers.
It never left his side much like the little squirrel never left Michelangelo's side.
When Donatello emerged from his lab a few days later carrying a most peculiar, small and somewhat round creature, Splinter was absolutely certain something very strange was going on. One or two strays being found by his young sons was a coincidence. But surely a third was absurd. Especially since this was Donatello, his head-smart little genius who was always tampering with whatever strange gadget they found out in the sewers.
It wasn't hard to believe his kind, cheerful youngest would take in a lost animal. Even Raphael -whom Splinter knew had a kindness to him that he hid from side for fear of being seen as weak, despite never looking down on his brothers for their kindness- even him Splinter could believe to take in a lost pet. But Donatello...
His genius little boy revered logic and science and would deem animals nothing more than a distraction or source of chaos in his ordered and controlled environment. One would think Michelangelo was enough chaos to deal with.
When Splinter asked his son, he couldn't even explain his actions or where it had come from, similar to the other two. "It just felt right." he had said. Donatello did not let his emotions guide him. Logic ruled his mind.
Thus Splinter dove into research, trying to find anything similar happening before with the humans, noticing the eery resemblances between his son's pets and life as he remembered up above. Unfortunately he could only conclude that the strange substance that turned himself into a rat and made him lose even more than he already had at the time, this same substance that had changed four little turtles into the growing mutant turtles he was now living with, must have had another side effect. A severely slowed reaction to the "forming" that happened when a human baby left its newborn state and became more aware.
His sons had formed Daemons.
Knowing what these "pets" truly were now, Splinter began noticing other things. His sons' daemons never shifted form as most childrens' did until a certain age of adolescence. Almost as if this was already their true form, already settled. Splinter could only conclude that it was another side effect from the substance that mutated them.
When he deemed it time to explain he was relieved to find his sons responding well to the knowledge for the most part. Michelangelo was filled with glee as he now realised how deep of a connection he had with the little squirrel that followed him around. Raphael wa mostly confused but seemed to accept it and Donatello was clearly curious and would no doubt try to find out everything he could about daemons and how different they were for humans.
Leo however only looked concerned and worried. Splinter could imagine the questions going through his son's head. Why now? What does it mean? Will this be dangerous? Will my brothers be safe? And most importantly, clear as crystal in his eyes as he stared at his brothers and their daemons.
Why don't I have one?
"Be patient, Leonardo. Your brothers did not all receive their daemons on the same day. You too will soon have one, I am sure of it."
True to his word, several days later Leo too got his daemon. But unlike his other sons, the forming of Leonardo's daemon was witnessed by them all.
They were sparring one morning. Donatello and Michelangelo were already sitting down, breathing heavily when Raphael gave an uncharacteristic shout of surprise before jumping away from Leonardo whom he had been sparring with. Leonardo looked startled at his brother before looking where his brother was then pointing.
Just behind Leo's shoulder was a strange glowing orb of light. When Leo moved, it moved right along with him. As quickly as it had appeared, the light suddenly fizzled out of existance and a small creature landed on Leo's shoulder.
Light coloring, enormous ears for the creatueres full body, small pointy face and a bushy tail to finish it up.
Leo carefully raised his hand, the small creature ducking ever so slightly as it observed his hand trying to deduce his meaning, before completely relaxing aginst the fingers rubbing just behind its ear.
As his sons crowded around Leonardo to look at it more closely and introducing their own daemons to the newest addition, Splinter slipped out silently and unobserved. In his own room he uncovered a hidden photo of himself as a human and the beautiful crane bird standing beside him. He wondered is she yet lived, after their bond had broken so suddenly when he was changed from a human into a rat.
He could only miss her and be thankful he still remembered her. The substance must have protected him from the breaking, preventing him from going catatonic as so many humans did when the bond was broken due to outside intervention or too great a distance.
It would never happen to his sons, this he vowed. This miracle happening in the next room, these daemons for four mutated turtles... it was a miracle and Splinter would protect it with his life.
End
Daemons are the physical manifestations of the souls of humans. Although dæmons are a separate identity to their human, together, both are considered to be one entity. Therefore, both the human and dæmon will die simultaneously, if one is killed.
Humans and Daemons thus share a deep connection, a bond that binds them together allowing the sharing of thoughts and feelings as both are the same entity. This bond can be broken without death though it is not common to survive the "Breaking". If survived, the daemon loses all sentient thought and became nothing more than a feral animal. the human falls into a catatonic state as if the will to live has vanished. "Breaking" is possible by stretching the distance between human and daemon too far causing it to snap. Other occassions involve near-death experiences.
Please keep in mind these are my own version of the rules regarding Daemons. Daemons are originally from His Dark Materials, book series and will have different rules.
Animal list:
Michelangelo - Squirrel
Raphael - Tazmanian Devil
Donatello - Pika [looks like a combination of a rabbit, guinea pig or vole]
Leonardo - Fennec Fox [Because Leo is my favorite he gets the cute little fox]
Splinter/Hamato Yoshi - Crane Bird [elegent bird, seemed to fit him in my head]
I wanted to keep the turtles' daemons fairly small to further set them apart from humans.
Someone on AO3 pointed out to me that a Squirrel was probably a bad choice for Mikey as in the newest series he's scared of them. I didn't remember this when I wrote this thus for now I'll keep him with a Squirrel but it might change if ever there come future stories with Daemon ideas.