If the enlargement is mostly on the front of the ankle joint they are called osselets. They usually do not give a horse any problems if he is moving sound after racing. You see them most often in race horses that have long and/or low pasterns (commonly known as 'coon-footed'). Many ex-race horses have them. If you are serious about buying this horse, I would have a couple of quick x-rays taken to make sure they are osselets and not bone chips that would need to be removed.