Hmm, interesting...i wish there were more info on them...if it is some sort of mutation, or comes from certain breeds etc. it is interesting to know they were named after a tb stallion and my horse is a tb/qh cross...
The origin is an Arabian stallion Ben D'Or. Since arabians are part of the thoroughbred breed the spots show up there. And because other breeds contain tb from way back, we see it in almost every breed. A friend had two chestnuts mares, one much darker than the other with small fingerprints of darker hair randomly scattered from the shoulders back.