You probably aren't surprised that David Aber named this “Horsebox.”
To make this even better, here's the other side — with a real horse in the background:
He sent these photos on April 19th, which is the same day that his camera says he took them. Thanks!
Update (March 12, 2021): Four blocks east is Theraputic Riding of Tucson, with lots of horses — and horse murals. (Click there to see it on the Tucson Murals Project blog.)