Fluttershy's brother, first announced at a comic book convention this year in San Diego, will very likely appear in Season Six of My Little Pony Friendship is Magic. He's very likely a Pegasus pony.
If Flutterbro is a younger sibling of Fluttershy, perhaps, PERHAPS, he is an exchange student who moves to Ponyville from Cloudsdale.
When Flutterbro first comes to the Ponyville Schoolhouse, he is introduced by the teacher Miss Cheerilee to the other students, including Scootaloo, Apple Bloom, and Sweetie Belle.
Flutterbro lacks his own cutie mark, and the Cutie Mark Crusaders try to remind him of the importance of ponies having cutie marks.
Season Six might also see Flutterbro interact with the draconequus Discord, Princess Cadance's baby, the Apple family, the Pie family, etc.
Flutterbro trains under Rarity and Photo Finish as a would-be model, learns cooking and food preparation from Pinkie Pie and Mr. and Mrs. Cake, weather control from Rainbow Dash, animal care from his own sister, and other stuff from Twilight Sparkle and Spike.