Thank you all, the bump is a hardened hematoma that had formed to protect a torn tendon we are working towards treatment.
You should see your veterinary as soon as possible. It can be caused by health problems and injuries that cannot be seen with a naked eye, and only your veterinarian can diagnose the problem.
If it is an orthopedic injury, one of the best results in its treatment can be achieved by the use of stem cells in injured place as soon as possible. The fresher the injury is, the better are the results of stem cell therapy.
Consult your veterinary about this case, and, if stem cell therapy is appropriate for your case, you or your veterinary can contact us, and we will provide you the necessary treatment injection. Usually, it is enough with one stem cell treatment injection, and the first results can be seen after two weeks. The treatment will also reduce the inflammation and pain of your horse and improve its mobility.
Could be several things, including: a bog spavin, arthritis, capped hock, inflammation from a knock from his other leg, infection, or tumour. Really only your vet can identify and diagnose it.