I think stress could deal with it a lot, the pony I was talking about was a riding school pony, as soon as he had the yeast, and the one on one, it stopped, and when they stopped the yeast, still didn't come back...
Especially if it was a long time companion? Is there any chance she can go out with another 'big' horse, and not just the mini? She might miss the big horse mutual grooming etc... or I might be crazy- anything is worth a go though!
LOL sorry, was just throwing the grass thing in there as a 'maybe' just in case.
Hope you manage to find the solution!