Did Dimetrodon have fur? I've heard that animals like Dimetrodon didn't have scales like a reptile.

Interesting question!

Quick answer: As far I know we don't have any direct evidence for what Dimetrodon's skin was like but it is usually depicted with smooth skin or scales.

Dimetrodon was part of a group of animals called Synapsida, which is distinct from reptiles based on the placement and patterns of holes in the skull. This means that Dimetrodon is part of the group from which mammals eventually evolved, but itself was definitely not mammalian. Some early synapsids are thought to have possessed boney armor embedded in their skin, which may have looked superficially like scales but are not formed in quite the same way. It is unclear when fur first evolved in the fossil record, but the earliest undisputed fossil imprint of hair is from the late middle Jurassic (164 million years ago), which is long after Dimetrodon would have gone extinct in the Early Permian (272 million years ago).