Jumbo the Elephant
Jumbo the Elephant is a unique SLASH'EM monster. He only appears in real zoos, and he is the only unique monster without his own level.
As a pet
Given his high base level, damage, and zero MR, Jumbo could be a nice pet during middle game if you can spare a scroll of taming or a charm monster spell, although he doesn't possess any resistance other than stoning. He doesn't have any to-hit bonus, so he still has difficulty dealing with monsters needing +x weapon to hit, such as statue gargoyles, vampires, shadow wolves. Since he is not fast, he is likely inferior as a steed when compared to ki-rins and Pegasus.