Climate/Terrain: Any land
Activity cycle: Day
Int: Non- (0)
# App: 2-12
# Att: 3
SD: see below
Size: H (20' long)
Morale: Steady (12)
XP Value: 2,000
This is another of the quadrupedal dinosaurs of the ornithischian order; it is a relative of monoclonius, styracosaurus, and triceratops. The name pentaceratops means "five-horned face", but the aggressive herbivore really has only three true horns--the two apparent horns on the sides of its face are only cheek bones. Its shield (sometimes reaching over much of the back) and horns give its head AC 2, while the unprotected parts of its body are AC 6. If it charges it can trample smaller creatures, inflicting 2d10 points of damage.
Originally appeared in Monstrous Compendium Volume Three: Forgotten Realms Appendix (1989).
Penataceratops was about 8 m (27 ft) long, and has been estimated to have weighed around 5,500 kg (13,000 lb).
Individual Triceratops are estimated to have reached about 7.9 to 9.0 m (26.0–29.5 ft) in length, 2.9 to 3.0 m (9.5–9.8 ft) in height, and 6.1–12.0 tonnes (13,000-26,000 lb) in weight.
So they are pretty comparable in size.
I'm not sure what the three attacks represent, but I think it would be safe to just go with a gore like the triceratops.
Comparing game stats to triceratops:
Tri had 4 more HD, did more damage (1-8/1-12/1-12) and had a deadlier trample (2-12), and was 4 feet longer. Otherwise, they were identical.
Other than that, I'm not finding much to differentiate them from triceratops in game stats. Perhaps we could give them greater natural armor and/or a chance to avoid critical hits due to the larger "shield"?
I could rant for a while 'bout dinosaur stats... the unfortunate truth of the matter is that triceratops was the only ceratopsian with full bony infill of its frill, so if any of them deserved light fort, it'd be them. But the MM made dinosaurs boring.
If we wanted to reflect the science, and we totally don't have to, the frill would be used for display. So, a racial bonus to Intimidate checks and frightful presence. Maybe the skin on the frill changes color to resemble giant staring eyes?
The three face horns of the pentaceratops should really count as one attack (1d6) since the dino can't aim them separately and they are spaced closely anyway. I doubt it will bite (I don't know of horned belligerent herbivore biting to attack or defend). The two 1d10's could be the cheek horns though these are actually shorter than the other three, but maybe the dino applied more force when tossing its head sideways.
Demiurge's got a good idea about the frill. And, in fact, in Walking With Dinosaurs, the torosaurus frill does sport eye spots.
As a sidebar, ceratopsians split into two groups: the short-frilled family (Triceratops) and the long-frilled family (Torosaurus).