Just an idea, from buying a Virginia horse. Have her selenium checked. He was extremely low, got very sore in work. Not actually head bobbing, but not right.
With the test results, we got him the selenium shots for immediate raising his levels, with selenium and Vit E top dressed on his feed. He was retested within a month, showed a great improvement, but not up to normal levels yet. Took some time, but finally got him normal.
His action, stamina, smoothness in movement, all visibly improved with higher selenium levels in the body. We were not working him much at that time, but the few outings showed us he was getting better.
Selenium is supposed to be in short supply along that side of the nation, and working horses use up their stored supply. For us, horses who sweat regularly, especially mares, need more attention to their selenium and Vit E, than non-working horses or horses used lightly.
I know you get the Pair out and they work. Probably sweat every outing, often heavily. This means you have to give them additional, to keep the selenium and Vit E at normal levels.
Oddly enough Selenium helps make the muscles work easier, so fatigue in fit animal, doesn’t set in as soon. We noticed that with our VA horse. He came to us, with regular work driving, being ridden, supposed to be quite fit in his work. He really tired quickly though, and we sure were not working him very hard. His Pair partner wasn’t even sweaty. Not as fluid of movement in work, seemed sore muscled when work was done. Some of that we put down to shoeing changes, after upping his shoes 2 sizes to really fit correctly.
With a previous driving mare, we had lots of muscle soreness, bad heat cycles, terrible back soreness. We did the selenium test to rule things out. We were sure she had a lameness problem we could not pinpoint. Turned out with her EXTREMELY low selenium that her reproductive cycle was affected, along with the muscle soreness. She also got the shots, while we learned a WHOLE bunch about Selenium as a helping mineral in the body. We had a sharp learning curve on the selenium to get things readjusted in our feeding set up. The other mare, fed exactly the same way, had no selenium problem. But she was not sweating much in her ridden work. Not working hard, like the driving horse getting conditioned.
Getting the pony mares tested for selenium deficiency would be a good first step. At least you can rule it out as a problem. You can keep on checking for other reasons. We had one horse who just had a busy head, sometimes bobbed it, just never really still. No lameness, just irritating as possible. She was that kind of horse.
If Maya is very bobby, she could be lame, someplace. We went thru a lot of “nope, not that” ideas, before we stumbled on the selenium problem. Video tape might help pinpoint a location, since with tape you can slow her way down.