"He balked" is something you might hear an equestrian say quite often. Or "When she's in a bad mood, sometimes she balks about going into the canter."
Balking is refusing to move forward, which may or may not be accompanied by moving sideways or backwards instead. A horse may balk because of a scary object ahead, because of pain, or just because he doesn't want to do what he's being asked. Balking is a common way in which a horse will test an unfamiliar or experienced rider.
The UK spelling is "baulking," and Brits may also use "napping" to describe this behavior.