Dimetrodon meaning "Two-measures Tooth" is a carnivore meat-eating pelycosaur mammal-like reptile, an ancestor of the mammals, that lives during the Permain Period, long before the dinosaurs evolved.
The Dimetrodon only appears in the original The Land Before Time, when Littlefoot, Ducky, and Petrie hide from one crossing their path. As the Dimetrodon passes it flicks out it's tongue like a snake or monitor lizard and hisses. There are two inaccuracies with this encounter. One, is that Dimetrodon did not live at the same time as the dinosaurs. However, many dinosaurs and other prehistoric reptiles from different times, places, and periods of time appear in The Land Before Time; which could excuse this. And two, is how it sticks its tongue out to taste the air like a snake or monitor lizard. Since it was a relative of mammals and not monitor lizards and snakes, Dimetrodon would not have done this.