The Night Pigsy is a common creature spawning throughout the surface when nighttime starts (around 10:34 PM ingame-time), especially on any type of Grass, also Dead Grass and Savannah Grass, but only in complete darkness.
The Night Pigsy can also be found in caves on the Fossil layer or can spawn on most kinds of blocks from this layer under the right circumstances in darkness.
This night creature will start to burn with a blue flame when hit by sunlight (around 5:11 AM ingame-time). It will then perish without leaving any trace (nor drops/loot).
Just like Pigsies (their yellow-colored relatives roaming the world during daytime) Night Pigsies will also drop Leather, Red Mushrooms, Crystal Shards, Bones, Globs of Goo and/or sometimes Sinews when being killed.
As Pets, additionally to what they usually drop, 1-3 Pigsy Droppings will also be added as a pet-harvest every time - a good fertilizer that can speed up the growth of your Crops and/or tree-Saplings.