Well, Chandler would be within the Hohokam culture area. Now, certain phases of Hohokam pottery were known for repeated painted renditions of animals, birds, humans, covering the outer portion of the pottery vessel. Here is one such example of a repeated human figure commonly found on Hohokam pottery:
https://www.flickr.com/photos/bclee/3361944444
One notices that the figures are all the same, but, since they are hand painted, they are not each identical to the other. Your piece is of course stone.
Now, someone pointed out it looks like they were made from the same stencil. However, looking at the gap between the lower left leg of the lower horned toad, and the body of that toad, that gap seems a little wider then the gap between the lower left leg of the upper toad, and its body, suggesting to me that there is at least that slight difference between the two, and therefore not made from the same stencil. I could be mistaken, it's a judgement call, but actual measurements could be made to confirm or not.
So, repeated figures are common on Hohokam pottery. It might suggest Hohokam for your rock. Certainly, painted designs will survive in your arid climate, on rock, as well as on pottery. What is truly odd, at least in my experience, is the green color. I have never seen that color pigment on their pottery before, but I'm no authority. So, perhaps someone with greater knowledge of Hohokam culture and artifacts could help you here.
And we certainly cannot rule out Joshuaream's suggestion that the stone fragment could be modern in origin....