Since we were out of town, we had the volunteers take her to the vet to be watched in case things went downhill quickly. Luckily, some bute and stall rest stopped the buckling. Neurological issues were ruled out and we were left with what appears to be a growth and conformation related tendon issue. We had Hamilton's vet, Dr. Ted Vlahos, review the videos as well, and the consensus is that she likely just grew too quickly, and her tendons were unable to keep up. It's not uncommon in yearlings.
Unfortunately, with her ringbone from starvation and her very upright conformation, there are really no treatment options that can help her. Normally, one can actually cut that tendon without problems, but in her case, that surgery isn't recommended due to her build and ringbone. So, for now, she's going to have to get through this on her own. She is on controlled and limited turnout and exercise, and a controlled diet meant to slow down her growth (meaning we'll be keeping her leaner than some would like). Unfortunately, that's mandatory for any chance at a semi-normal life for her. She already is expected to be a pasture pal, companion type horse anyway, so her long term prognosis hasn't changed, but we want to give her every chance we can!
At one month of rehab, Erin seems to be doing well. Her left hind is back to normal, standing properly and landing as it should. Her right hind remains a bit more upright so we've added a shoe to encourage her not to stand on her toe, however with the conformation and ringbone it's hard to tell if it's truly struggling or if she's just that upright. At this point, we suspect the tendons have caught up as she runs and plays normally, without any sign of lameness or pain, and we see no other symptoms such as swelling or buckling. She and Hamilton will remain in a small paddock near the barn with the option to come in to a stall overnight throughout winter, however, as we monitor them both to ensure they are doing the best they can.