Pachycephalosaurus

A thick-skulled herbivore whose domed head is a living battering ram, used in bone-jarring contests and against any predator foolish enough to crowd it. It lowers its head and charges in a dead-straight line. Herds graze the uplands and butt heads in thunderous rutting duels heard for miles, and a bull driven from the herd becomes a foul-tempered loner that treats everything which troubles it as simply one more thing to ram.

Ram: bludgeoning, melee 5 ft, +1 to hit, 2d6+4 bludgeoning. Skull Charge: after moving at least 15 ft it slams for an extra 2d6 and knocks the target prone and back 10 ft on a failed Dex Quick save (stunned 1 round on a critical fail), cooldown 2 rounds.

Bone Dome: its reinforced skull ignores the first 2 points of damage from any single blow and it resists being stunned. Hard-Headed: +2 to saves against charm and fear. Senses: Twilight Sight, Keen Smell.