Archaeologists have found evidence that these dogs were companions ... One of the Xoloitzcuintli’s most unique features is its lack of hair. However, not all Xolos are hairless.

The mutated form doesn't do that properly, so the hair falls out. The AKC recognizes four hairless breeds: the Chinese crested dog, xoloitzcuintli, Peruvian Inca Orchid and American hairless terrier.