Unknown, but possibly Sharpteeth
The Nightmare Longneck (or the Longneck Monster) is a dragon-like Diplodocus who appears in the episode The Lone Dinosaur Returns in Chomper's nightmare. It is a dream representation of Doc, and it appears in the Mysterious Beyond.
During the course of the nightmare, Chomper hides under two ground spikes to escape what is chasing him. The Longneck Monster then appears and smashes the ground spikes to pieces. He doesn't understand why the Lone Dinosaur wants to hurt him, because he is good. It ignores his pleading and attempts to smash him with its tail, but Chomper gets out of the way just in the nick of time.
Chomper flees and climbs onto an even bigger set of rock spires, and again he says that he will never be a mean sharptooth. The monster still does not listen to him, and destroys the spires, throwing Chomper to the ground. He continues to plead the monster to believe him, but it ignores and swings its tail at him. Before it can squash him, Chomper awakens and yells in fear.
- It appears in the same location that the Spinosaurus lives in The Land Before Time XII: The Great Day of the Flyers and in the TV series, in one of the desolate areas of the Mysterious Beyond.