While it doesn't happen every ride, my horse has a tendency to take a funny step usually at a walk though sometimes at the canter (O_o) where it feels like he's stepped in a hole with one of his hind legs. His back end seems to drop out from underneath me and he'll raise his head to counter balance and the next step will be normal. During an hour long ride, it might happen 5 or 6 times or not at all.
I don't think he's ever done it at the trot. He doesn't seem at all bothered by it happening. He's fitter now than he's been since I've owned him (five years) and I haven't noticed its occurrence increasing or decreasing.
Any ideas on what it could be? And how to make it stop? It's pretty disconcerting!
The horse here they did Estrogen injections on, I believe it was about 4-6? That fixed him, totally. He is no longer here, so I don't know if the problem ever returned. He was here for about a year after the injections though.
I have to agree that it's a likely loose stifle as I've also had this happen with more than one horse. Although you say that the horse is fitter than ever the looseness can happen from an injury, possibly when the leg was overextended in some direction maybe from a slip on wet/slick ground. Can happen while turned out. It can be a very frustrating issue and the stifle needs to tighten up, means very controlled work, no circling, no extended gaits, no jumping, no galloping, no extended trot, use collected gaits. It can hard to "see" for a vet unless it's quite bad and happening frequently.