Discord and Bill Cipher made multiverse bet called Friendship Project where they accidentally open portals and send random MLP characters across multiverse for friendship assignmenta.. Applejack was one of bunch..She got through portal and because of multiverse laws now she is human..She found her surroundings strange but what she found more strange was blonde 7 years old boy swinging alone being ignored by everyone 😭..She immediately understood that she have to keep him safe and be Big sister she has to be.