My elderley neighbour walks his Fell pony to the village shop, and the pony has panniers on. Thats about 13hh, so someone must make them for bigger ponies. I have some for my llama, proper llama ones, and they are huge, and would probably fit a pony.
We have a broller collar we use on our Dales and Fell ponies to chain harrow. This is a french collar, like a cross between a collar and breast collar I suppose. I have used the harrows with her pulling in her breast collar. She is a big strapping girl and can do it whatever she is wearing.