Dimetrodon is an extinct genus of synapsid which lived during the Permian period, 280-265 mya.
Though they may appear to be dinosaur-like, Dimetrodon is not a dinosaur. Dimetrodon is more related to mammals than to true reptiles. Feel your temples on the side of your head, that is a character humans share with Dimetrodon, a temporal opening or temporal fenestra. This serves as a location for the attachment of the jaw muscles.
See also: Edaphosaurus
(Picture source 1, 2, 3)