Rarest domestic horse hybrid

The rarest horse hybrid was a unique three-species specimen documented as follows by none other than Charles Darwin in his book The Variation of Animals and Plants Under Domestication (1868): "Many years ago I saw in the Zoological Gardens [of London] a curious triple hybrid, from a bay mare, by a hybrid from a male ass and female zebra. This animal when old had hardly any stripes; but I was assured by the superintendent, that when young it had shoulder-stripes, and faint stripes on its flanks and legs. I mention this case more especially as an instance of the stripes being much plainer during youth than in old age". This triple hybrid's father was thus a fertile male donkra (male donkey x female zebra hybrid), a great rarity in itself.

Because male donkras are normally sterile, some researchers have questioned whether the triple hybrid was truly a three-species hybrid, claiming that its father couldn't have been a bona fide crossbreed. However, a spontaneous mutation might have resulted in such a rare individual, just as although tortoiseshell cats are usually female because of the condition being sex-linked, rare spontaneous mutations can occasionally create male tortoiseshell cats.