Washed Out
A horse that becomes so nervous that it sweats profusely. Also known as "washy" or "lathered (up)."  

Weanling
A foal that is less than one-year-old that has been separated from its dam.  

Wheel
Betting all possible combinations in an exotic wager using at least one horse as the key. 

White
A horse color, extremely rare, in which all the hairs are white. The horse's eyes are brown, not pink, as would be the case for an albino.

Wire
The finish line of a race.  

Withers
Area above the shoulder, where the neck meets the back.