1. Presumably (I'm guessing) the base creature has to be considered an animal (not vermin) and only base forms are eligible (i.e. not giant forms, luckily: a giant were mammoth). In addition to this, I strongly suspect that marine only and cavern creatures are excluded.
In fact, I think werebeasts can only transform into [MUNDANE] creatures -- I've never seen were-unicorns and the like.
Werebeasts actually have their own list of eligible animals hidden within the coding. They don't extract anything from any editable RAWs. That's why there are "wereapes" when "ape" isn't a creature in Dwarf Fortress, and similarly "werechameleons" when there are no chameleons.
Most of the time I ignored werebeasts, not bothering with them besides killing and\or throwing them into magma. Recently I decided to actually try and make some interesting stuff - werebeast militia, traps, the usual.
Then I realised that I have no idea how werebeasts work, so to the wiki I went. After reading about werecreatures I still have some questions.
1. It is said that werecreature may assume form of any mammal or reptile. How exactly is decided if animal is eligible to be, er, 'curseable', I guess? For example, bats are considered vermin, but there are giant bats, and even description explicity says they are mammals. Is a werebat possible to create?
2. How exactly is created new hybrid form of werecreature? Humanoid body, but covered in scales\fur and with a head of an animal? What about size or hybrid form? Wereelephant must be bigger than weredeer.
3. If werecreature gets wings in it's hybrid form, will it get the ability to fly?
4. Wiki mentions that when you become a werebeast, you get bonuses to strength and agility. Exactly how big are these bonuses and do you get them permanently or only for a duration of your transformation?
5. Let's say we have a werecrocodile that is weak to bronze. Then it infects someone else with werecurse. Will the new werebeast gain weakness to bronze, or his weakness will be generated independently? How exactly does this weakness work?
6. IIRC in Adv mode you learn werebeast's weakness by talking to people. How do you learn about it in Fortress mode? Answer is probably 'by repeatedly impaling it on spikes made of different materials', but maybe there is less time-consuming way?
I'm sorry if these questiions were already answered somewhere else, but I didn't find anything.
1. Already answered above. Werebeasts don't borrow raws from regular animals, but instead have their own list of werebeast creatures with werebeast-specific raws.
2. "Twisted into humanoid form." So basically, think of more modern depictions of lycanthropy, such as werewolves with wolf-like heads. Size varies on creature, but all werebeasts have a minimum size of 80,000.
3. There are no flying werebeasts. Lot of rodents, reptiles, canines, and assorted (elephant, mammoth, ape, etc), but no avians.
4. The boost is only available in werebeast form, and it doubles all your stats. However, due to the size increase of werebeasts, the double in strength is more effective than that of vampirism.
5. The new werebeast will gain weakness to bronze. The weakness is strain-specific. However, keep in mind there can be two different strains of werecrocodile, each made by a different god (or sometimes even the same god) and thus with a different weakness for each. Each werecrocodile will spread their respective strain, however, and thus their own weakness and not the other strain's.
6. Honestly, werebeasts are a lot less dangerous in fortress mode so their weakness is irrelevant. You can just atom-smash the thing if you want it dead, or you can use it to create a werebeast militia.