Designed by Mark Bolitho (see comments) folded from 15cm kami. Final model ~9cm long.
This model is in an origami set from Muji called Dinosaur Origami. (Of course Dimetrodon isn’t a dinosaur at all, it’s a pelycosaur and more closely related to modern mammals than to “reptiles” or birds.) Unfortunately there’s no accreditation for the designer(s) of any of the models (as well as the Dimetrodon, there’s Pteranodon, Triceratops, Ankylosaurus, Tyrannosaurus and Brontosaurus) or indeed any copyright information whatsoever.
The Dimetrodon is the most challenging of the models in the set, and took a good hour. I messed the feet up a little; once you get paper-slip on the corners of flaps like that it’s very difficult to retrieve the situation. Still, I am fairly pleased with the results.
There is another Dimetrodon in John Montroll’s Prehistoric Origami: Dinosaurs and Other Creatures which, although looking fairly similar in the end result, is formed in a very different way. Such is the magic of origami!
The Kitchen Sink 
I forgot to mention that, contrary to Robert Lang’s thoughts on origami diagramming conventions, the Origami Dinosaurs instruction book relies entirely on diagrams, with no verbal instruction at all.
Mark, I was looking for images of the origami muji sets and came across your blog, its several years too late, but I was the author. Great you managed to make the model.
Thanks for the comment Mark, I’ve added attribution to the original post.