Leah, I don't care if the horse has antlers. If he's neither tender nor lame, then the bump or prominence has no medical significance, and you should not be looking for anything to "fix" it.
As to the identity of the bumps you mention: in all likelihood, you've just noticed the upper heads of the semitendinosus muscles, which form fleshy bumps on either side of the tailhead, extending from the notch at the dock forward to about a third the distance forward to the peak of croup. If the bumps you mean are between one-third and one-half the way forward, you're seeing the upper heads of the femoral biceps. It is, however, 100% certain you're not seeing the sacrotuberous ligament, as that is the "basement" tissue and would be very unlikely to be visible in a live horse under any circumstances, even if injured.
The sort of mistake you're making is one commonly made by people who have no real experience with anatomy, "real experience" being obtainable in only one place, and that is, around the dissecting table with an actual horse carcass that you can touch and manipulate. The surprise is how three-dimensional it is -- in other words, how THICK -- how MANY layers -- a real horse's muscular body has. No book printed either on paper or on acetate sheets can convey this adequately -- hence your mistake.
We do teach this class, and you could come and join in, Leah, as I am sure you would be a good and eager student. Next go is up at Dave Elliott's in October -- you can give him a call at (403) 394-4172 if you want to enroll -- I THINK there is still space in this class; or you could join in on our Equinology full-body class, or the skeleton class, here in California this coming November. To enroll for either of those, please EMail email@example.com. Best wishes -- Dr. Deb
Thank you Dr Deb. My second guess, after paging through more books, was the Semitendinosus Muscle. I readily admit I am NOT a scholar in this area!
I guess I was curious/concerned because I have never seen this before and wanted to be sure it was not something that would lead to a long term issue. Granted no one can predict the future but if you read my post and said, "Oh Leah, please have you vet out, that can lead to xyz in most horses with this" I would have been more prompt in dealing with it.
Oh how I would LOVE to attend. I will look over your schedule.
You really should come to the Southeast sometime! Georgia is a lovely state!