Another for the 100 animal challenge.
Dimetrodon means "Two measures of teeth". Basically what the 280 million year old bones tell us is that these and creatures like them were very mammal like compared to the reptiles today. The two tooth reference means that it had canine/tearing teeth and teeth for grinding in the back, a cornerstone feature of mammals today. The sail which is connected directly to it's spinal vertibre is theorized to have been used to help warm this predator up, and to allow it to maintain body heat to hunt longer. I envision this creature lounging in the sun all day and becoming active to hunt at night when smaller mammals came out. While it is commonly agreed this is not a reptile despite being placed with dinosaurs popularly I did reference it's colors from the frilling lizards of today and made the frill on it's back colorful to be useful for mating rites.