This past August such a thing happened to the five year-old horse Benny the Bull. He was found to have a bone chipped off in his ankle late in the season, and it had to be surgically removed. At this time he was preparing for the Breeder's Cup Sprint, but was obviously pulled out of the race due to injury and retirement.
Benny the Bull was the top sprinter in the nation, went the 2008 season undefeated, and won the Eclipse award for best sprinter. Because of his amazing speed and having been durable enough to last through four years of training, being put up for stud was an obvious road for the horse. International Equine Acquisitions Holdings Stable even stated shortly after his retirement that Benny would be standing at stud in the 2009 season.
However, at the Eclipse ceremonies Iavarone, his main owner, declared in the his acceptance speech for Benny that the horse would be on the track again.
This is a section of ESPN's coverage article: Benny the Bull coming out of retirement
"After examining Benny the Bull, Dr. Bramlage said he felt his injury had healed sufficiently and that he could come back at 100 percent again," said Iavarone. "We actually made the final decision to bring him back about two weeks ago and decided to wait to announce it until tonight at the awards ceremony. But under one condition: that he won the Eclipse. If he won, we wanted to bring him back to defend the title. If he had not won, he would have stayed retired. Fortunately, Benny got the recognition he deserved by being named the champion."
~Mike Welsch of the Daily Racing Form
With that being said, one of our times best runners is making his way back to the track. Personally I can't wait until his first start to see this old guy (by racing standards) continue to strut his stuff and own the sprinters for another amazing year.