California Code, Revenue and Taxation Code - RTC § 5703

“Racehorse” means each live horse, including a stallion, mare, gelding, ridgeling, colt, filly, or foal, that is or will be eligible to participate in or produce foals which will be eligible to participate in a horseracing contest in California wherein parimutuel racing is permitted under rules and regulations prescribed by the California Horse Racing Board. “Racehorse” does not mean or include any horse over three years old, or four years old in the case of an Arabian horse, that has not participated in a horserace contest on which parimutuel wagering is permitted or has not been used for breeding purposes in order to produce racehorses during the two previous calendar years.