This isn't really a bug but a feature request. The Mod is working exactly as it should.

It would be useful for mod compatibility if the three prop entities that support EntityHorse as a beast of burden were to instead check for AbstractHorse. That would allow Mules and Donkeys to also be able to be used as well as any other mod entity based off of AbstractHorse.

Alternatively if this isn't possible it would be useful if we could register 'horse-like' entities that should be valid beasts of burden for the purpose of those props

I'm apparently an idiot, this already works.

I'm going to go ahead and clear this because I've clearly missed something.