Workout

A workout is a timed sprint over varying distances prior to a race, meant to get a horse fit enough for competition.

Scratch

A horse is a scratch when injury or other necessity causes it to be withdrawn from the race.

Rabbit

Also known as a pacemaker; this is a horse with no expected chance of winning but is used to prompt a quick pace on the front end of the race to the benefit of a stablemate who is a closer.