The Doctor likes horses, they have a lot of common sense; he knows because he speaks horse. It’s a useful skill. He can ride too, which is fortunate, because out West, if you need to get somewhere quickly, horseback is the only way to travel.
Needing to track the gunslinger, he borrows a horse and rides out of town. Susan, as the horse prefers to be called, is an excellent mount, fast and sure-footed, intelligent, and has quite a mouth on him. The Doctor laughs to himself. The preacher would blush scarlet if he knew what his horse was saying.