Does it count as bestiality if...

McGonagall cat and Sirius dog lol imagine the offspring

I think if they were both in human form it would be ok but i can imagine it being all kinds of wrong if one of them suddenly transformed

The same has been asked in regards to True Blood and the shifters and were's. Guess it depends if their in shift or not.

I would say it would depend on what their "true" form was...if they're an animal who shifts into a human..than yes...but if they're a human who shifts into an animal..then no.

The exact definition of bestiality is: Sexual relations between a person and an animal, including intercourse, masturbation, cunnilingus, frottage (rubbing), anal penetration, or having ones genitals licked by an animal. So, if an animigus in animal form sleeps ('m assuming here you mean sex?) with a non-animagus, then, yes, it would be bestiality.

However, if two animagi in animal form sleep together, according to the definition, it wouldn't be bestiality, since they are both in animal form.