Dimetrodon, as with many of the animals we discuss here, is not a species but a genus. Therefore, there are actually numerous species under the genus name of Dimetrodon. Happily for us, though, we have the benefit of having years of research under us and can look pretty completely at most of the species. The typical iconic Dimetrodon species is D. grandis.
D. grandis was named early in the 20th century around 1907. This is about 30 years after the first 4 species were discovered and named, however, this remains one of the most easily recognized animals in paleontology. Another species that is we see a lot of is D. limbatus.
However, not all of the heads are like the two we've looked at and the sails found on the many species of Dimetrodon are certainly not identical. In fact, they are heavily varied, just like dinosaurs can be heavily varied. Here are a few more examples to look at: