There appears to be a regular stream of articles about the latest "hybrid species" to be discovered. Even reputable science journals seem to regularly dig them out with continual expressions of surprise.
Interspecies hybrids are usually sterile. Because chromosome pairs need to be reasonably close to align and transfer DNA. But the key word is usually it is not always. For those who insist on species-ism, new ones are continually being formed in front of our very eyes. It is evolution in action right now.
A species is only a group of organisms that tends to interbreed at a point in time. There may well be an ideal mate for each animal, but if the first choice is not available, then some do "choose" to mate across our specious divides. There are some weird individuals around :)
OK, it is probably useful to think in terms of species if you are categorising the diversity of life at this moment. But when looking through the eons of history then that model of nature is not so useful, it is more instructive to think of the transmission of molecular patterns rather than species.