So glad it isn't anything needing surgery! My mare was kicked in the hock, before I got her, and it caused a huge lump on her hock. It didn't go away in the 9 years I had her, but it became stiff at the age of 24. Before that, it never caused even the slightest issue. But vet said it was only because it was on a joint and she was already getting arthritic, otherwise it wouldn't have ever been a problem.
Best of luck with you and your horse :)