Horses that dig wells...
Huh, I've never heard of this before... Wild horses off North Carolina live on Shackleford banks, a small 9-mile long island, and they will dig themselves watering holes. I didn't know horses would do that. According to the website, http://www.shacklefordhorses.org/about.htm these horses have very unique territorial behavior... but it doesn't explain exactly what those behaviors are.
There seem to be a lot of wild horses off the Eastern coast, there is an island off Georgia that has them, and of course Asseteague and Chincoteague islands... and I'm sure there are others. But these horses don't seem to be managed by BLM like mustangs are.
The pic on this page, of the "flaxen" stallion http://www.shacklefordhorses.org/stories/colors.htm is actually a bay silver dapple... rare color and he is a GORGEOUS example of one, one of the most striking silver bays I've seen.