I am obliged to report that which is reported, but not to believe it.
—Herodotus, The History, VII, 152
Note: Claims that hybrids can be produced from this highly disparate cross require confirmation.
Horse-cat hybrids seem to be somewhere between rare and impossible, but at least one was seriously reported in the news. The following brief announcement appears on the page 2, column 1, of the August 9, 1913 issue of the Bismarck Daily Tribune, a newspaper published in Bismarck, Dakota: (Access source)
Valley City, N. D., Aug. 8—A freak colt with ears and tail just like a cat is an object of curiosity on a nearby farm and several offers have been made the owners by showmen who wish to exhibit the monstrosity. The colt is of the regular farm type with no particular breeding and its peculiar appearance is entirely unaccountable.
Obviously, this cross would be extremely disparate—that is, if we could believe it occurred in the first place. But the authenticity of this report has never been verified, nor do reports of other such hybrids seem to exist. Indeed, one must wonder whether a tomcat would even be able to mate with a mare. There are, however, various cases where small males have succeeded in mating with much larger females. For example, it is definitely known (documented on YouTube) that small dogs sometimes copulate with cows while they are lying down and dog-cow hybrids have been repeatedly reported. But until more horse-cat hybrids are reported there seems to be no reason to believe this hybrid ever existed, nor to pursue the topic of this particular cross any further.
The following is a list of reported cat crosses. Some of these crosses are much better documented than others (as indicated by the reliability arrow). Indeed, some might seem completely impossible. But all have been reported at least once. The links below are to separate articles. Additional crosses, not listed here, are covered on the cat hybrids page.
By the same author: Handbook of Avian Hybrids of the World, Oxford University Press (2006).
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