When there has been tendon damage (generally in racehorses that have broken down) it is a way of 'fixing' the damage by the use of heat. Not sure of the exact process but it's quite an old fashioned technique. I am sure there will plenty of websites that will explain in detail. We have an ex-racer who has been fired and you can see the white scarring on his legs.
That is has been either pin-fired or bar-fired on the front tendons. It leaves very obvious scars, and is/was done to promote blood flow to the tendons after injury or strain.