Underlay

A horse that is going off at lower odds than it deserves based on performance.

Scratch

A horse is a scratch when injury or other necessity causes it to be withdrawn from the race.

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.