Dimetrodon was a carnivore synapsid that lived during the Permian period. It is not a true dinosaur, but is actually as reptile likemammal. It lived about 250 million years ago, and has a close relationship to mammals. It is well known to the public.
During its time it was a dominant predator, and was about 3 meters in length. It had two types of teeth, one which was used for for shearing, and the other which was shaped more like canine teeth. It had a large tail and was a quadruped which walked on four legs which sprawled. One of the most noted features of the Dimetrodon is its sail.
Most paleontologists believe the sail was used to regulate the body temperature of the animal. The large surface area would have allowed it to effectively be cooled or warmed. It could also have been used to scare off potential predators, or as a method for attracting mates. Because Dimetrodon was a synapsid, it may be related to the many mammals which exist today.
Even though Dimetrodon existed 40 million years before the dinosaurs, it resembled them in various different ways.