I'd like to read a lot of stories about women who agree to be trained as ponies either consensually or semiconsensually. A lost bet, a paid contract, to please their husband, or for the thrill of it
I prefer modern non-fantasy settings, or historically 18th or 17th century plantation style pony girls pulling plows
There are a lot of stories about ponygirls at Sir Jeff's site, which stopped adding stories in 2014.
You can also check Grommets Plaza which has a section devoted to Pony and Pet Girls.
There are probably a few on SOL. The problem is even a quick search for ponygirl is going to leave you trying to manually sort out furry stories from human pony play.
ElSol flirts with it in El Ojo De Malicio (which is a sequel to El Sobador), but it's in the background.