It’s a proto-mammal.

Tetrapod animals (those with four limbs) are all thought to be related.

This group includes two main divisions amniotes and non-amniotes. Amniotes produce young in eggs with four distinct layers (one of which is the Amnion) which makes them well-adapted for surviving on land. Frogs and Salamanders are examples of non-amniote tetrapods.

Amniotes are a large group which are divided into groups based on the number of openings in their skulls near the temples. The two surviving groups in today’s animals are synapsid (one pair of openings) which includes all mammals. And diapsids (two openings) which includes all modern reptiles and birds.

The Mesazoic Era (the age of the Dinosaurs) was 250 million years ago (Ma) to 65 Ma.

Dimetrodon was a synapsid that lived about 295 Ma. So it’s not really in the right evolutionary family to be a dinosaur, and it lived long before dinosaurs.

It superficially resembles a large lizard (but in reality it’s a closer relative to mammals than any true dinosaur).