I’m nearly as far from an expert on infectious diseases as it gets, but - and if anyone who knows about influenza reproduction can chime in - I remember reading that influenza has incredible abilities to mutate wildly and recombine. The analogy was like, if human reproduction is like taking two decks of cards and randomly shuffling half of each deck together, then influenza is like taking any number of decks, randomly chopping up and re-splicing portions of random individual cards together, as well as resorting all of them back together without any regard for whether the results are going to even produce anything that can live or not. But the reproductions and randomizations are so voluminous that it doesn’t matter - at least some of it will stick.

In other words, in addition to the wildly rapid mutation capabilities these viruses have - if you have animals that are carrying more than one strain of influenza simultaneously, those two or more strains can produce hybrids.

But again: citation needed.