Carnotaurus
By DiBgd at English Wikipedia, CC BY 2.5, accessed here
[Proof that evolution has a sense of humor, but the results can still mess you up pretty bad. The cannibalism mentioned is based on the related Majungasaurus]
Dinosaur, Carnotaurus CR 6 N Animal This slender biped stands tall on long, clawed legs, and its arms have atrophied into little more than nubs. Its short, deep skull is crowned with two small horns and its jaw is filled with serrated teeth. Its body is covered with tubercles and scales.
The carnivorous dinosaurs called carnotaurus combine bone-breaking strength with remarkable speed. They are thin and leggy creatures built for sprinting. Rather than tackle similarly sized megafauna, carnotauruses specialize on small game—although to a creature as large as a carnotaurus, “small” includes creatures of human size. They prefer to strike with speed, snatching a lone target in their jaws, gulping it down in a single motion, then retreating from possible retaliation.
Carnotauruses are typically solitary creatures, but gather in large numbers to display and mate. Although both male and female carnotauruses have horns and rugose skulls, those of the males are thicker and are used in ritual shoving matches to establish breeding hierarchies. Carnotauruses are not above cannibalism, and ritual combat sometimes gives way to fights to the death if a combatant shows signs of weakness or ill health. A carnotaurus grows to about 30 feet long and weighs about 3,000 pounds.
Carnotaurus as Animal Companions
Starting Statistics: Size Medium; Speed 50 ft..; AC +1 natural armor; Attack bite (1d8); Ability Scores Str 15, Dex 21, Con 8, Int 2, Wis 13, Cha 12; Special Qualities low-light vision, scent, sprint. 4th-Level Advancement: Size Large; AC +3; Attack bite (2d6); Ability Scores Str +4, Dex -2, Con +4; Special Attacks devastating bite, grab, swallow whole (Small)