Today the NPS keeps a stable of 6-8 mules, all females and often sisters. They spend the winter on the grounds of Mount Vernon. (President Washington was a huge fan of mules and bred them.) The only two places on the canal where they're pulling tourists today are Georgetown and Great Falls.
Mules were originally chosen to pull the canal boats because they were cheaper than horses, less prone to getting sick, had tougher skin so less likely to get harness sores, were more sure-footed, and can pull a heavier load than a horse. If well-taken care of, a mule could pull a canal boat for up to 20 years. They actually lived on the boat with their familes in the front cabin.
I was told these mules came from Virginia and Kentucky. The mules that are trained to ride people down into the Grand Canyon come from Tennessee.