Przewalski's horse
"Mitochondrial DNA research has shown that Przewalski's horse is not the ancestor of modern domestic horses. [There are] a number of consistent differences in their appearance. Przewalski's horses, unlike domestic horses, shed their tail and mane hair once per year. Thus it is clear that Przewalski's and domestic horses are very closely related and have in the past interbred, but the fixed chromosomal number difference between them indicates that they are distinct populations".