An entity of light summons the Fairy Tail guild to Equestria to assist the Council of Friendship in a war against an ancient evil force. (For Fairy Tail, this takes place after the defeat of Zeref and Acnologia. For My Little Pony, this takes place one year after Twilight Sparkle became ruler of Equestria.)
The Natsu team, doing a job, ends up in a world full of ponies. Unfortunately, with them have also come a group that aims to plunge that world into total darkness. The Natsu team joins forces with Mane 6 to stop them.
When Gunfire (One of my other OC's) buys a mirror with a weird bird on it, he falls into it one night and stubles into a whole other universe with a whole other cast- as a Human!
Crossover of Fairy Tail and Mlp Equestria Girls. It is just something I came up with. Rated T for safety. Suck summary I know. I can't come up with any good ones