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!