In the 1960's, units painted the concrete donkey at the park near the officers club before they deployed to Vietnam.
The 36th Engineers, who made the donkey, didn't want to deface their creation, so they decided to paint the unit crest of the 36th on a rock near the front gate. (inset photo)
Today units completing a rotation at the NTC can request to paint a rock and add it to the Painted Rocks
And the donkey, well he went with the engineers to Vietnam.
https://www.amazon.com/National-Training-Center-Images-America/dp/1467127957