Desert Horned Lizard
You can see from the shape of its body why many people used to call these guys "horned toads", though of course they're not related to toads at all. Nevertheless, the flat body and those amphibian looking rear feet do give it that appearance.
The flattened body also helps to make this type of lizard difficult to eat, because you'd need a pretty wide throat to swallow it.
This was the third of these lizards that I saw in the park, and was jumpier than the first two - but the fourth one was even jumpier and I only got one photo of it. After first running away, this one at least let me get my tripod set up so I could take a few photos in natural light. It's rather a peculiar lighting, since this was in a small gully surrounded on three sides by large red rocks reflecting the late afternoon sunlight, which gives an unusual pinkish tone to the shot.