Dimetrodon

Dimetrodon was an anchosaur from the Permian Period. The animal was 3 meters long, weighing 250 kilograms, with a sail to attract females. It was a nasty predator.

Sexual Dimorphism
However, there was a thing about these creatures. The males are bigger than females and were sexually dimorphic, which means occurring in or representing two different forms. This is a common thing sometimes.