Bump on my horses shoulder?

There's a little hard bump on my horses shoulder... Its really small... like the size of a skittle... or smaller. Is that normal? My other horse had a bump on his leg (A lote bigger tho) and so does my friends horse (My friends horse also has a bump on his leg) The bump on my horses shoulder was there when i bought him i think, but it doesnt seem to bother him. Its just weird... its really small but it sticks out and its really hard? Like his bone or something? Its not on his other shoulder, just his left one. Should i just ignore it?