A horse with a hard lump on the top of its hock is said to have a hock?
It is called a capped hock. A horse's hock is the same as a human ankle. Most likely if your horse has a large bump on the top of its hock, it has been injured in some way. On a horse that has not been injured there should be what appears as a small bump on the back of their hock, this is called the point of the hock. I would suggest consulting your vet about any bumps you find on the front of a horse's hock.