Luna's Night Guard[]
Lauren Faust and M.A. Larson tweeting about bat ponies.
The Night Guard debuted in Luna Eclipsed (S2E4) as Luna's personal bodyguards. They stuck out from the other ponies as having different features than the normal ponies. While they were interesting, it wasn't until Equestria-Prevails created Midnight Blossom when the bat ponies really hit it off. The Night Guard once again made an appearance in the Nightmare Moon timeline in The Cutie Re-Mark (S5E25-26) once again as Luna's bodyguards.
When the bat ponies finally became a trend on /mlp/, their popularity skyrocketed and began to show up en masse on other sites.