Apprentice

A new jockey who has not ridden a certain number of winners in a specified time period.

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.

Main Track Only

A horse entered for main track only (MTO) is one hoping the weather forecast or track conditions will move the race off the turf course and onto the main track. They are scratched the morning of the race if the conditions stay dry.