There's a list of mythical beasts mentioned on page 16 of Thurman translation (Click on the definition to see an image I have found of the mythical creature):
Brahmas: Creator gods.
Sakras: Indra, king of gods.
Lokapalas: Kings of the 4 quarters, protectors. Also Wikipedia.
Devas: gods. Wikipedia.
Yaksas: forest demons.
Gandharvas: Heavenly musicians. That are birds.
Garudas: Magical bird. Which, by the way, is the national symbol of Thailand
Kimnaras: Horse head, human body.
Maharagas: Mythical serpent race, of which I couldn't find any images.