The Doctor has found himself stranded in Equestria. A way back will take time. A lot of time. During the interim, he needs to blend in, so he uses the chameleon arch to turn himself into a pony, complete with a (mostly) pony mind. To a large degree, this will be the story of an out-of-place stallion trying to settle into Ponyville (with the possibility of some adventuring in the future). Of course, nothing's ever easy for The Doctor.