"They like to work. They need to work." (11.6)
Emilie's grandpapa believes that horses must always be working. While it may seem that he's being careless toward the horses, from what you know about Joey, wouldn't you agree?
We were mere workhorses, and treated as such. (12.6)
Even though horses like to work, they don't like being treated as workhorses. There's a fine line between a working horse and a workhorse, and the main difference is respect. Sometimes the soldiers refer to themselves as workhorses, too, because of the dehumanizing working conditions they're suffering through.
"The only reason you're here is because you were brought here." (13.7)
Joey may be a horse, but he was drafted as much as any human. In fact, wouldn't you say a horse is drafted to do everything he has to do? He has no choice or say in the matter, really.