The dimetrodon is not aggressive. If you do attack it, don't worry, they're very slow and quite easy to take down.

I have found many of them inland on the lava island. 5-9 present at any point in time. Coord: 30,65 approx.

I found most of these guys in swamp areas. Can be used to regulate temperature for incubation

Encountering this this it looks like a baby spino and Croc combined but I find some good level (Level 10-25-almost level 30)(on the island) in the redwoods.