Disturbances in the magic of Equestria causes rifts between worlds. One unlucky hiker stumbles into one such rift and finds himself in a new body and in a new world.
AJ Smith, was approached by one Rebecca Haddington, who was reporting her brother Tom missing. While investigating the disappearances, AJ ends up buying a brown Stetson from the Merchant.
When John Radderson ended up as an anthropomorphic alicorn Princess Twilight Sparkle, she set off a chain reaction that transformed her classmates into her pony friends.