But in Barbaro's case, well-to-do owners Gretchen and Roy Jackson are hoping for a full recovery.
"They are optimistic that if this is a successful healing he will be a successful stallion," Sweeney said.
Barbaro's sire, Dynaformer, commands a $100,000 stud fee.
Barbaro was the odds-on favorite to win the Preakness and set up a Triple Crown try in the Belmont Stakes. But a few hundred yards out of the starting gate, he took a bad step, his leg flared out grotesquely and he veered sideways before jockey Edgar Prado pulled the powerful colt to a halt.
Later that night he was vanned to New Bolton and surgery lasted most of the afternoon on Sunday.
"He looked pretty bright, I guess you can't ask for anything more," said trainer Michael Matz, who guided Barbaro to an undefeated record before the Preakness.
"I'm hoping for the best, I'm very optimistic. It's going to be a long time and we just have to take it day by day and keep our fingers crossed."
Barbaro has been receiving antibiotics and pain medication, and is able to move around -- or even lay down -- in his stall if he chooses.
"For this to be successful, the horse has to be able to stand during the healing stage," Sweeney said. "Lying down also would be advantageous to healing."
Barbaro sustained a broken cannon bone above the ankle, a broken sesamoid bone behind the ankle and a broken long pastern bone below the ankle. The fetlock joint -- the ankle -- was dislocated.
Richardson said the pastern bone was shattered in "20-plus pieces."
The bones were put in place to fuse the joint by inserting a plate and 23 screws to repair damage so severe that most horses wouldn't have survived it.
Horses are often euthanized after serious leg injuries because circulation problems and deadly diseases can arise if they are unable to distribute weight on all fours. Also, money is a factor.
For extensive surgery and recovery, it could cost "tens of thousands of dollars," Richardson said. Many owners choose against trying to save a horse with a serious injury.
Matz was encouraged after his visit, but was left wondering what might have been had his horse won the Preakness.
"It would have been great if he could have won the Triple Crown," Matz said. "He tried. He won the Derby, he got hurt.
"We've had horses that broke down before. It's something that happens. Sometimes you can't save them and sometimes you can.
Hopefully, this will be one of the times we can save him," Matz said.
Inside the center, apples, carrots, peppermints and flowers continued arriving for Barbaro. On a white sheet of paper next two dozen roses was a note: "Thank you for doing all you can to save Barbaro. America is so thankful." It was signed, Silvia, Miami F.
