It really depends on where you live. Different countries have some disciplines that are unique to that country.
Dressage is universal and she looks like she has has an uphill build, so that's option (I think most countries are holding Pony Dressage classes now.) Plus I'm a huge beleiver that Dressage benefits every horse, even if they're competing in other disciplines.
However if you live in the UK;
-If she's under 133cm (Around about 13.1hh) you could Show Hunter Pony classes as she fits that type
-Or you could do Native Pony/M&M classes for her respective breed.
If you live in Australia;
-Show Hunter Pony classes, regardless of the height as there is no upper height limit.