I'm thinking he was part of a gang called The Bat Bastards, who pretended to be vampires for fun and terrorized town. Vampirism being, obviously, a hoax made up by non-ponies to insult bats, which are the best type of pony.
Gone to jail a few times, the last time which he got to spend a lot of time with an elderly pony, Firefly, who was doing a life sentence for a few counts of arson and had become quite religious in jail, like people sometimes do in real life. Firefly usually prayed holding an amulet which ended in Night Night's hoof after they passed away, the acquaintanceship leaving him quite enchanted with the power of faith.
A few weeks after leaving jail, Night Night was thinking about starting to settle down, but a dead body drained of all it's blood was found in town and suspicion immediately came to his gang, which scattered in all directions. After a few planets hopping, he ends up in Archibast.