If memory servers from long ago and MeRP stuff, they had some "Fell" animals, not jut the filying Fell Beasts.
Basically I guess the idea was that anything could be "corrupted" - Elves into Orc's etc so no reason why almost anything could not be twisted into monstrous form.
So a Fell Bear, or Lion/Rhino/Horse etc should be still achievable withot being completely out of canon as such.
Although if I remember it, the Fell Bears were to the North - monster polar bears basically.
Still choose an animal - Fell it up a bit in size, job done.
Please no badgers, ferrets, or squirrels though - well, maybe the squirrell..
I like the Fell Eagle idea as well
