Arsinoitherium
This was an Oligocene beast that looked much like a rhinoceros, except that it had two massive horns side by side on its nose. Like a rhinoceros, could charge for double damage. Low or prone opponents would be trampled for 2-12/2-12 hp damage. Treat Arsinoitherium 9-HD rhinoceros, as per the Monster Manual, except that its horns do 4-16 hp damage total.
Originally appeared in Dragon Magazine #137 (1988).
Yes, I agree with Freyar on 9d8. The Eocene saw a burst of large mammals to fill the void the dinosaurs left. Relative brain size is low compared to modern mammals however, so a low Cha and modest Wis is fair enough.
We can compare the bite damage from the fangs to those of a hippo, but scaled-down since its fangs are smaller.
Cha 2 strikes me as unfair. What's the Cha of a rhino?
Powerful charge and trample for Arsinotherium seem like no-brainers. Perhaps we could do something to capitalize on the double horn? Improved Critical as a bonus feat, maybe?
I'll take Augmented Critical. The gore damage must be high on charge as that's how it deals the majority of damage with those horns. 9HD with 2d6? What about 2d8? And then it's charge does double that so 4d8? That would work. 20hp damage is quite a bit for a low CR critter. Gotta make those PCs hurt a bit!