Yeah, you guys were reading my mind accurately. I was specifically thinking of the gryphon, though.
Yes, shame that D&D didn't go for that spelling.
Getting back to the flavour and tactics, it looks pretty good. I'd use a modifier to the Handle Animal check rather than include a reference to Magical Beasts. Of course I'll want to add a couple of personal tweaks...
A bear-sized elephant with foot-long tusks as straight and sharp as spearheads.
An onco is a small, aggressive relative of the elephant. Oncos wander the jungles in small family units, normally made up of a single male, 1-4 females, and possibly young. Solitary onco are generally lone males in search of mates. Oncos are herbivorous, grazing on the jungle floor, which clears paths for other big animals.
Onco are too aggressive to make good mounts or beasts of burden (add +10 to the DCs of Handle Animal checks to tame or train an onco). Their tusks are fine ivory, worth around 100 gp on the open market. [
The SRD Elephant doesn't have a price for its tusks, suggesting we should cut that bit from the onco. However, if we do include it we'll need to modify the Treasure, e.g. "Treasure: 1/10th goods (ivory tusks, see below)".]
COMBAT
Oncos are highly territorial. They usually try to scare away intruders, but may charge anything they think threatens them. An onco fights by striking with its trunk and tusks. If it hits with its trunk, it often tries to pin its opponent to the ground and run them through with a tusk.