Deliberate mistake: In the final race of the film, Seabiscuit performs a flying lead change at the gallop while crossing the finish line, in the famous shot taken underneath his neck. Horses normally only switch their leads around turns while racing; it's something done to relieve pressure on the leading foot. From taking the shot over and over again, the horse playing Seabiscuit was probably tired out and a little tender and began to perform lead changes in other places in an attempt to be more comfortable.
Seabiscuit (2003)
1 deliberate mistake - chronological order
Directed by: Gary Ross
Starring: Jeff Bridges, Elizabeth Banks, Chris Cooper, Tobey Maguire, David McCullough, Paul Vincent O'Connor
Factual error: Many of the main characters are using modern-day binoculars (black, plastic) throughout the entire movie.
Tick Tock McGlaughlin: No more match races for this little horse because frankly they're all out of matches. Who's he gonna race, Lazarus? I pity these horses.
Trivia: The jockey at the crude outdoor race track that Pollard watches as a kid is wearing the same clothes he wears later on in the movie.
Answer: They probably do, it just isn't shown. Alternatively, they could have died in-between the time we see the young Red and Tobey Maguire. That could be a contributor to all his anger and rage.