Stegosaurus

A plated herbivore that turns its back on attackers to bring its spiked tail to bear, a sweeping club of bone spears that can punch through a predator's ribs. The plates flush with color in warning before it strikes. It browses low ferns near the herd and never strays far, ponderous but immovable, asking only for room - and answering anything that crowds it with a tail that swings like a war-flail.

Thagomizer: piercing, melee 15 ft, +1 to hit, 3d6+4 piercing (a sweep of the spiked tail). Tail Sweep: it lashes all creatures behind it for 2d6+4 and knocks them prone on a failed Dex Quick save, cooldown 2 rounds.

Plated Back: bony plates and bulk turn aside flank attacks. Defensive Pivot: always keeps its tail toward the greatest threat. Senses: Twilight Sight, Keen Smell.