On Dec 14, 2017, at 6:20 PM, Jeff Zeitlin <xxxxxx@freelancetraveller.com> wrote:

Simple question, straight up: Is it plausible that canids (or
xenopseudocanids) of some sort could be domesticated and bred up to a size
that would make them usable as cavalry mounts?

Sure. Orcs did it :-P 

Dogs have been used as draft animals, and have been saddled to give children rides, so it’s entirely plausible that one could be bred or geneered to be large enough to ride.