Dimetrodon vs. Saltwater Crocodile. Comparable size and weight (perhaps the croc is slightly larger), but Dimetrodon's sail might give it an initiative advantage in cool weather/early mornings. Plus it just looks so badass prowlin' under the red sky of the primeval world:
Croc wins. Really big males can be 6 metres long and weigh 1300kg, and I'm sure there's a reason why they have existed relatively unchanged since similar times. Dimetrodon they reckon grew up to 4m. Croc would probably see that big sail and ambush it if he was hungry enough.
You can do anything, but not everything.