I am obliged to report that which is reported, but not to believe it.
—Herodotus, The History, VII, 152
Pig-horse hybrids are very rarely mentioned in the literature. Indeed, there seems to be but a single, brief news report about such a hybrid. Nor is the alleged hybrid even pictured. And such a cross would, of course, be extremely distant. So this cross is very poorly documented indeed (as reflected in the low reading on the Reality Thermometer at rightbelow).
Pigs are quite indiscriminate in their choice of mate, as documented in a variety YouTube videos, which show them mating with a wide variety of other kinds of animals, and by the various crosses listed for the domestic pig on the pig hybrids page. So it is not surprising, in itself, that a boar might choose to mate with a mare if, say, she were lying down or some other opportunity offered. The question is whether a hybrid could actually be produced from such a mating.
The following brief notice is the only report of a pig-horse hybrid that extensive search has thus far revealed. It appeared on page 5, column 1, of the February 11, 1892 issue of The Globe-Republican, a newspaper published in Dodge City, Kansas, U.S. (access source):
One of Stubbs Bros.'s high bred mares gave birth to a monstrosity this week. It was a perfectly formed colt in all respects except the head, which has a striking resemblance to that of a hog. This portion of the freak is on exhibition at the grocery store.
By the same author: Handbook of Avian Hybrids of the World, Oxford University Press (2006).
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Branches of Biology