There are a few questions that should be answered before you throw an animagus into a story. What animals can people turn into? How much should their personality reflect their animal? How does being an animagus affect their magic, personality and appearance? How should you convert human-years to animal-years concerning size and abilities? Can they turn into more than one animal?
Add the obvious follow-up questions and we've got ourselves a small essay.
First of all, keep an open mind and then decide on rules to limit the story and make it realistic. An ironic statement perhaps, but you can't throw every ingredient into the cauldron and expect it to do anything but explode. And most people would prefer if everyone didn't run away from their story. So start by choosing ingredients.
Now, what animals can people turn into? If we follow Rowling we see everything from dogs to deers to insects. Can they turn into smaller animals? Ticks? Bacteria? And how about the other way? Elephants? Whales? Which brings us to another question, water-dwelling animals. But most would set a limit here that they're comfortable with and not many would argue with it.
What is more of an issue is whether magical animals should be allowed. Rowling don't have them but they do add colour to a story. There are of course issues here as well. What kind of magical creatures should be possible? My first issue is intelligence. Where should you draw a line? Mermaids, Giants, Goblins, House-elves, Centaurs, Veela? They are all intelligent enough to speak a language, most even understand English (I won't even mention werewolves and vampire since I don't consider either an actual creature but simply cursed humans). Should their intelligence be a factor that makes it impossible to turn into them, they are magical creatures, are they not? Most would probably feel reluctant to use these as animagi, perhaps even argue that they are not animals at all, but simply intelligent life forms, much like humans. However, using my own argument with their ability of intelligent conversation, shouldn't this rule out snakes as well? Parseltounges can speak to them and most fics give them quite a bit of cunning.
My point is that the line isn't as clear as you may think. Another line that you should approach is even harder to pinpoint. Should an animagus be able to turn into a Boggart, or how about a Dementor? They are certainly creatures but especially a boggart should be very hard to turn into since it isn't even clear whether they have a form at all. On a side note I have an image of Voldemort being a boggart, and his goal would be to appear as himself before anyone when he transforms.
The bottom line is: make a list of creatures that you feel doesn't fit in with what wizards should be able to turn into, and those you have doubts over, and if you don't plan on using any of them in your story then make a rule that makes it impossible to turn into them, and come up with a reason why, and your story will have that much more credibility.
Another is whether a persons personality should be a pointer to which creature they are. Could the brave Harry Potter turn into a hare? Could the meek Neville Longbottom turn into a dragon? Could the devious Draco Malfoy turn into a hippogriff or a unicorn? On the other hand, mentioning Neville brings up another issue, he was sorted into Gryffindor. There have been several suggestions that he carries an inner bravery that comes out when things matter the most. Couldn't this inner personality be a hint of his animagus-form? And if so: couldn't others have the same inner personality? Couldn't Draco have suppressed his so much it would only surface through his animagus-form? With this logic anyone could be anything even if it have to suit their personality since they could have other, hidden sides.
This obviously brings us to the next question. We all know how prolonged exposure in an animal-form give the animagus traits from that animal even when they turn back. A good thing to decide would be whether a person should change their personality after discovering their form, especially concerning magical creatures. If Draco Malfoy was a Unicorn would he become pure and kind after his first transformation? Would it take a longer time, would he have to be in that form for a longer period of time? Should it have no effect at all unless he is in his form?
Also, what about physical changes? Didn't Sirius Black look like a scruffy dog? Did he always look like that or was it a side effect to discovering his form and using it often? Would my example of Draco as a unicorn give him a horn on his forehead?
Another aspect, when dealing with magical creatures, is of course magic. Should a dragon-animagus have an easier time casting fire-spells? How about the magical resistance? Miranda Flairgold gives a good example of this in her story 'A second chance at life' and the sequel 'Changes in a time of war.' If your animagus-form has an inclination to certain elements (fire, ice and such) or spells (protection, illusion, you get the picture) then should they be easier for the animagus to cast in human form? Rahkesh's obsession with lightning isn't a coincidence. - On this note I have an image of Harry Potter being a Phoenix and the night Voldemort cursed him he survived by being reborn. Maybe I'll write a fic based on that but the idea has most likely been used by others, which shouldn't deter me, or you.
Another issue is the fact that different animals and creatures age, grow and die differently. The first concern is lifespan, I doubt that Rita Skeeter only lived for a few months (if even that, how long does a bug live?) and then died of old age. But how does it work? Do you convert the years from human to animal, like we do with cats and dogs, or do most animals look really old when the humans has reached thirty? Can the animal form die before the human does? If we assume that you decide a middle age on both the human and the animal they can turn into and then divide them equally to get, for example: ten human years for every animal year. How do you deal with wizards who lives form hundreds of years? And what about animals that don't stop growing until they die. Most snakes are like that, right? What if Nicholas Flamell (sp?) was a snake-animagus? Wouldn't his form be huge? Miranda Flairgold takes up this point as well in the aforementioned fic by her. She first decides that there are vampires who are thousands of years old, and then that dragons never stop growing until they die. Combine the two and the result is one huge reptile.
Lastly, can you turn into more than one animal? If so, what if they clash with each other? A cat and a mouse in the same body should result in a fierce inner conflict, especially if the animagus is affected by the animal-personality. If you can turn into more than one, the question of how they are chosen is brought forth. If we assume that you can't chose your form, that it was somehow chosen for you at birth, does that mean that two could be chosen as well? Should there be a balance between the two animals? Can you have one dragon-form and one rat-form? Can you have two forms of the same animal? Is there a primary and secondary animal? How much emphasis should be put on the conflicting effects two forms can bring, or should you just see them as advanced transfiguration and give them out like candy?
Again I urge you to read Miranda Flairgold's 'A second chance at life.' And remember, keep an open mind, question everything, rule out nothing before you've considered it and be curious.
