Sequel to My little Background Characters

Several months have passed since the defeat of Nightmare Moon and Derpy's Dalek dream. Derpy originally hoped this would end and she could have a finally normal life. However, the end isn't in sight. Discord could destroy Equestria, Changelings may ruin what Derpy hopes could be an "imperfect" day, and King Sombra could wipe out an entire empire. Then there's that strange stallion who shows up everywhere. Who is he?