Giant lizard
Giant bug of some kind
A giant flying fish who speaks spanish for some reason.
A panda
A lllama
A gelatinous cube.
What kind of things live in your underdark? What is the character like? You should come up with something to match.
Lots of bats in the underdark? Give him a giant bat.
Dwarven lumberjack? Log-rolls his way around on an amputee roper.