trotter

noun, A horse (or other quadruped) which trots; spec. a horse especially bred and trained to the trot (“trotter, n.” OED); [‘trot’: A gait of a quadruped, originally of a horse, between walking and running, in which the legs move in diagonal pairs almost together, so that in a slow trot there is always one foot at least on the ground, but in a fast trot one pair leaves the ground before the other reaches it, all four feet being thus momentarily off the ground at once (“trot, n.1.” OED)]

“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.”

“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.”