The Dimetrodon was a dreaded apex predator, which lived about 300-270 million years ago - thus 40-70 million years before the first prehistoric saurian. It was not a "real" dinosaur, but rather a prehistoric reptile which was surprisingly closely related to mammals. It preyed upon reptiles, amphibians and fish, which it tore into pieces with its many razor-sharp teeth. Its scientific name essentially means "two measures of teeth", because its teeth were of different sizes.
Fun fact: The Dimetrodon moved like a lizard, since its legs were on the side of its body.