Night Light is a carnivorous resident of the Everfree. Unfortunately (Or fortunately, depending) Cain happens to live out in her hunting grounds. Honestly, it’s just surprising she didn’t come across him sooner~
 
My favorite headcanon for most fanged ponies (Lamias, bat ponies, etc.) is that they inject a powerful aphrodisiac before feeding. In the event of Night Light’s feeding, that happened to bring out a much more.. Willing side of her food source. Naturally, she was delighted to oblige~