Summon forth your megaloambystoma. Or should that be *max*olotl? You decide.
The fact that this impossibly large ambystoma is fitted with an elaborate saddle strongly suggests it was created to be ridden. However, its presence in Pandæmonium means it was flawed in some way and deemed a failure.
How to obtain:
Raid -- Anabaseios: The Twelfth Circle (Savage)