Every now and then, your horse will bash into something, hit his hipbone, or bang his knee. Most of the time, these are minor injuries that will go away on their own.
A lump or bump is caused by trauma to an area that causes underlying swelling and possibly a broken blood vessel under the skin, called a hematoma. On people, these are signified by their color, and are called bruises.
If your horse comes in from pasture with a bump or a lump, first inspect it. He may have a little broken skin, but as long as there's no puncture wound, it is probably just a bruise. Apply a cold compress, such as an ice pack or ice bandage if possible, for 15 minutes to reduce the swelling. Repeat the process every couple of hours. If the swelling is around a tendon area and is accompanied by any kind of serious lameness, best to contact your veterinarian.
Simple bruises usually resolve on their own. Just keep an eye on it and use the compresses for the first day. If the lump persists or doesn't shrink, check in with your equine medical professional.