This isn't my horse ... but one of the horses on pasture board at my barn.
We discovered a very very small lump on his butt. It's maybe the size of a pea or a little smaller. But when pressed, it can move around a lot under the skin. There is no disruption to the fur, so I don't think there's a wound that needs to be opened and trained.
Any idea what this might be? Am hoping it's just something from a bite; it's a little too high (and very small) for a kick. It's pretty well up on his rump.
Is it hard, or "squishy"? It may be a small fat deposit, nothing to worry about. In general, (although not always!) a lump that can be moved around is more likely to be nothing major, where a lump that is attached is more concerning.