This is ridiculous - deliberately inflamatory and provocative. I suspect you are PETA troll. Have you ever actually been on the backside of a race track?
First of all - horse racing is legal, whereas dog fighting is not...and for very good reasons.
In dog fighting, the animals are trained roughly and cruelly to encourage them to be agressive - other dogs and small animals are maimed/killed in the training of these dogs. The express purpose of every dog fight is for the two competitors to maul each other until the loser is either dead or so wounded it is incapable of continuing. These dogs don't interact with humans in any kind of normal way. Their owners don't care about their dogs' welfare except as fighting machines.
In horse racing, the aim is to keep the horses sound, healthy and happy so they run well. The purpose of a race is to cross the finish line first, and come back to the barn in good order to race another day.
Horses are treated kindly from the moment they are born. They are socialized with humans and other horses as foals. They are trained gradually and introduced to their work over months of training. They receive the best of care - housed well, fed well, fussed over, conditioned to be at their best and then asked to do something they love doing in the first place - run.
In the process it is true that even with the best of intentions some horses sustain injuries. Sometimes, not often, those injuries are catastrophic.
Is racing perfect? Is there more we can do to protect these great animals?
Yes, we need to do more to protect all equine atheletes: show horses, race horses, pleasure horses, all of them. However, unlike the dog fighters, race horse trainers care deeply about their horses' welfare.