Is she neurotic or what?
I have a large pony(Welsh/TB) has some weird behavior. While she is galloping and playing around the pasture she tries to bite her sides. She also has reared while under saddle. She usually rearsa after she's been ridden for a while and she wants to be finished. I am starting to think she is "mentally challenged". Has anyone ever seen this?
Sounds like ulcers to me. Galloping around could cause acid to splash the sides of her stomach. She may be rearing because she's uncomfortable.
There are a lot of threads on treatment options. Maybe try a full tube of ulcerguard for a few days and give some tums and see if she has improved.
Ditto ulcers. If not ulcers, perhaps ovarian cyst/tumor.
But try treating for ulcers and see what happens. Also, keeping forage in front of them as much as possible is very beneficial.