I have been vaguely aware that mules were also a source of cordovan. I'll discuss this with my pal Greg Kramer, who buys a lot of horsehide from Horween, and report back to the forum if I learn anything interesting. I can only imagine that the mule population (at least in the developed world) has plummeted relative to the horse population since they are so seldom employed as draft animals nowadays. An amusing anecdote: Some years back I was at the San Fernando Valley Fair. There were some mules in a pen, and a nice city lady said to her toddler, "Oh, Timmy, look at the horsies." I couldn't resist myself and in the best rustic accent I could affect said, "Lady, them's mules."