Metriacanthosaurus was a Sinraptorid theropod. They appear in The Lonely Journey. After fleeing from a trio of Fast Biters, Chomper spots one running towards him. It turned out that there was a whole pack (herd refers to herbivores). However, they are stampeding and don't see Chomper. They were stampeding away from Red Claw.
After hiding from Red Claw; Chomper sees a young Metriacanthosaurus (that little brown smudge in the picture of the adults). He tries to teach it to play catch but it doesn't understand him (probably because he was talking in leafeater). It brings it's parents who chase Chomper, but he hides so they can't see him.