update: so we had a lameness vet evaluate our pony. He did a full exam, palpated everywhere, flexion tests, we trotted in hand, rode under saddle at all three gaits. His comments were 1) it's not nearly as bad as you made it sound on the phone 2) she passed flexion tests perfectly 3) she does favor the right hind leg but very very slightly, no need to xray or anything just yet because it is so minor. 4) pony moved away slightly when the area of the right side just below the si joint moving downward near right side of tail (SI/Pelvic area) was palpated firmly. He feels there may be an issue or pain anywhere from the right stifle up to the si area on the right hind. But still did not seem alarmed or have any sense of urgency in obtaining xrays just yet because like I said in previous post, we can't even see any difference in her gaits at all. She has even been picking up the left lead canter almost every time now, where a few months ago she never picked it up at all. He did say her soles were soft (again not horribly so) and her heels on the hind hooves were very low. I am wondering if these low heels on the back could be causing this right hind issue? Maybe just wishful thinking that if we get the heels under control then we can avoid injections or hopefully it means there isn't something more lingering in that si/pelvis area??
background: got pony in may, no issues at all, both leads there no bad habits....one month later, pony can't get left lead at all, resists going left at all and stop and backs up when asking to go left, tracking right she is perfect, september left lead is back and she gets it about 85% of the time, judge excuses pony from class suspects hind end pain or hoof pain but none of us watching could see anything to our eye, pony had just been trimmed the day before the show and did seem sore when walking on gravel.