Roundtail Horned Lizard

No problem! I think they only have the swollen tail like that in breeding season, and just the males. I wouldn't have known otherwise except I just found a guy that looked very similar to yours :) Always exciting to see something new too! These guys are a little different than other horned lizards that instead of running, they always sit out in the open but pull in their legs and tuck in the head and go into "pebble form" and they are ridiculously hard to see!

Luis, I actually think this is Phrynosoma modestum, the roundtail horned lizard. And it would be an adult in breeding colors. They are much smaller than the other horned lizards, and have the 4 equal sized horns on the back of the head like that. You're at the southern end of their range (http://www.iucnredlist.org/details/64078/0) and here's one that looks exactly like yours that I just found last week. The colors are a little different, but they are variable across their range and are the same colors as the rocks they live in. http://www.projectnoah.org/spottings/303676013