So, Greta had her follow-up appointment today. She's still super ouchy after the flex test of her LH. Vet did x-rays and ultrasound of both hind legs and saw the follow: a very enlarged medial suspensory ligament and the very beginnings of arthritis in the LH, and two bone chips from an old fracture on her RH, although the latter only causes minor stiffness.
We're treating it like an acute injury so as to play it on the safe side, which means 4 weeks at the least of stall rest, and hopefully another 4 weeks of pasture rest afterwards, if not just an additional 4 weeks of more stall rest (I'm really hoping for some pasture rest in there). She'll be getting turned out in the day pens like she was when she first arrived at the barn. They're big enough to walk around in but small enough so she can't get really fast and frisky.
If time has not healed things up enough after 2 months, then we will try injections. She doesn't have a lot of angle in her hind legs, so it's potentially a conformational fault that's causing this. The absolute worst case scenario is that she has degenerative suspensory ligament desmitis (DSLD), which would be caused by her conformation, but it's not too likely as it is something that usually only occurs in gaited horses. But it's still some frightening stuff.
All in all, not a good day. I feel awful. But on the plus side, hopefully the worse she will need is injections. Injections are doable. DSLD is NOT doable.
EDIT: DSLD is HIGHLY unlikely, and the vet said there's quite a few things she can do for it, but like she said: HIGHLY unlikely. It's quite mild, and there is only lameness when it's flexed, so once again the worse we can expect right now is monthly injections, and Lord knows I know plenty of ponies who get those and still do just fine. So, I'm more positive now!