Mastery ($3.60) won Saturday's Grade 2 San Felipe Stakes at Santa Anita by 6 3-4 lengths, but was quickly pulled up on the first turn and vanned off.
No word was immediately available on his condition. Mastery was able to walk into the van.
The incident overshadowed an exciting performance from the undefeated colt in his first start of 2016. Ridden by Mike Smith, Mastery led throughout the $400,000 San Felipe and pulled away in the final furlong to score a convincing win over 3-1 Iliad.
Term of Art, a 60-1 shot, finished third, followed by Gormley, Ann Arbor Eddie, Bluegrass Envy and Vending Machine.
Mastery set a solid pace of 23.50 seconds for the opening quarter-mile and 46.79 for the first half-mile. Gormley was within a length of Mastery to the end of the backstretch, with Iliad racing three-wide in a stalking position.
Gormley faded from contention on the turn, when Iliad moved into second. Mastery pulled away through the final furlong. Mastery ran 1 1-16 miles in 1:42.28.
Mastery is trained by Bob Baffert for Cheyenne Stables. The colt was unbeaten in three starts at 2 - a maiden race here in October, the Grade 3 Bob Hope Stakes at Del Mar in November and the Grade 1 Los Alamitos Futurity in December.
Those wins left Mastery as the top-rated 3-year-old in California going into Saturday's race.