Are dog-horse hybrids possible? Perhaps not. Such a cross would, of course, be extremely distant. Moreover, all in all, the available evidence for this cross is extremely thin (as reflected in the low temperature on the Reality Thermometer at right). But two such hybrids have in fact been reported, though it must be kept in mind that both of these reports are both old and uncorroborated.
The following is a brief account of a strange canine offspring foaled by a mare. It appeared on page 4 (col. 5) of the May 26, 1888 issue of The Progress, a newspaper published in White Earth, Minnesota (Access original)
It is not unusual for owners to put newborn hybrids to death, the reason being that they see them as unnatural monsters. White Earth is a census-designated place in Becker County, northwestern Minnesota.
In a much older account, the 17th-century German equine 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. 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