pacer

noun, a horse whose customary gait is the pace; a horse bred or trained to this gait, now esp. for harness racing (“pacer, n.” OED) [‘pace’, verb,: spec., a distinctive lateral gait, in which the fore and hind legs on one side move in unison, alternating with those on the other]

“What’s the difference between a trotter and a pacer?”
“Well the pacer, both legs go back and forth at the same time, and the opposite way with the trotter.”

“A trotter’s gotta stay on stride in a race. If he breaks stride then he’s disqualified, and the same goes for a pacer.”