Are dog-horse hybrids possible? Probably not. But one such hybrid has in fact been reported. The 17th-century German veterinarian Georg Simon Winters von Adlersflügel (1703, p. 138) mentions that he himself saw a horse with a long dog’s tail at Castle Dachau in Bohemia in 1648. His illustration of the animal (Adlersflügel 1703, Plate 26) is shown below.
Of course, his mere assertion that he saw such a creature is no proof he actually did, especially given that intensive search of the literature for reports of mammalian hybrids has brought no other claims about the occurrence of dog-horse hybrids to light. This total lack of corroborating evidence is not surprising given the great disparity between the members of Canidae and the members of Equidae.
It is true, however, that when horses are lying down, dogs will attempt to mate with them, as is documented in various YouTube videos. Moreover, Adlersflügel was one of the leading experts on horses of his day. One has to wonder why a respected professional would choose to fabricate such a story and even publish it under his own name?
The following is a list of reported dog crosses discussed on this site. Some of these crosses are much better documented than others (as indicated by the reliability arrow). Moreover, some are extremely disparate, and so must be taken with a large grain of salt. But all have been reported at least once.
Human Origins: Are we hybrids?
On the Origins of New Forms of Life
Cat-rabbit Hybrids: Fact or fiction?
Georges Cuvier: A Biography
Prothero: A Rebuttal
Branches of Biology