The bat ponies had very little screentime on the show, and these characters is a continuation of the whole legacy of background ponies getting a personality, which derived from the fandom since Derpy. Early artwork of bat ponies included one by the name of "Midnight Blossom" by equestria-prevails. However, the idea was driven to popularity with an OC created by Zee66 and the /mlp/ bat threads named "Echo" - a female bat mare with a navy blue mane, grey coat, and an apparent obsession for mangoes. As the idea caught on, more OCs were created, such as Florence, Moonstuck, and Midnight Snack, all with similar "bubbly" personalities like Echo - a stretch from the bat pony depiction in their debut.
Growth and Spread
By early August, the tag, #bat pony started trending on tumblr. More OCs were created, such as Moongazer, that further proved a point as described by Sethisto that they're "things that become way cuter when you put fangs on them", which makes perfect sense because Seth never goes to bed either.
On August 21, show writer M.A. Larson responded via Twitter with regards to whether the chariot pullers seen in Luna Eclipsed were intended to be an individual race of ponies, amongst earth ponies, unicorns, and pegasi. Larson responded that he personally believes its true, but is not sure if Lauren Faust thought differently. Lauren followed up on the question and agreed with the idea. She also mentioned that they were intended to have come from deep caves inside the mountains.
Threads on bat ponies have achieved general status back in June 2013. They are still allowed on /mlp/ despite the falloffs of mature threads after "the Scruffening", and have a sister general on MLPchan's /anon/ board as well. The degree of mature content involving bat ponies is not really any greater than the rest of the show characters, however.