Hematomas occur fairly regularly in horses. They are the result of trauma to a blood vessel, without a cut or breaking of the overlying skin, that results in a large swelling made up of blood. The literal interpretation of hematoma is a blood (hem) mass or tumor (toma).
These lesions usually show up on the upper body of the horse, over large muscle masses such as the chest (pectoral muscles) or anterior hind leg above the stifle (quadriceps muscle). I notice this problem more in young horses such as weanlings and yearlings.
The higher incidence in young horses could be due to their tendency to play recklessly, perhaps traumatize each other if they're running in a group, or possibly greater fragility of their blood vessels. Sometimes there is a mark on the skin implying a kick or hitting the fence, but I do see these lesions with no evidence on the skin indicating external trauma.
A common problem that can be confused with a hematoma is an early abscess (i.e., "pigeon fever" or "dryland distemper"). An early abscess will be firm and painful to touch, while a hematoma palpates like a balloon full of water and is usually not that sensitive to touch. In time, perhaps a week, the blood clot will contract and leave a pocket filled with serum and strands of fibrin.
In treating this problem, I believe there are two options. Some hematomas are so large and pendulous (the size of a soccer or even basketball) that they need to be opened and drained. Smaller hematomas can be left alone and they will reabsorb. I don't like to open a hematoma too quickly (less than several days), because you can cause continued bleeding from the traumatized blood vessels. It's pressure that builds up in the closed space plus the clotting mechanism that ultimately stops the bleeding. I would advise getting your veterinarian's opinion in treating each case depending on the circumstances.