Headcanon times: Bat ponies were normal guard ponies turned into a new race by Nightmare Moon. After she was banished to the moon, Celestia could never figure out how to undo the process. The new bat ponies were left to reintegrate into society, thought most married within the species due to being ostracized by the other races for being “evil”.
 
Most bat ponies followed Nightmare Moon out of fear or reverence (depending if they viewed Luna’s rebellion as righteous or not), but Starglow was different; she was downright cruel. As one of the first ponies to be transformed, she thought herself blessed by Luna, a race above all others, shaped by the princess herself.