There are a lot of bosses that drop pets and mounts such as Lady Blackhope on Unicorn Way and RattleBones also in Unicorn Way, but the chance of getting one is very rare. Also in Colousus Bulivard you can get a snow serpent in Golbber Kings Lair. That is all that I know.
Rattlebones (unicorn way) gives the mythsprite, General Achillies (Cyclopes lane) gives a blue ghost, foulgaze (old town) drops a dark fairy. Theres definetely more but those are of the top of my head
