This is a Vampire.
As it is a vampire it sucks the blood of other pigs and turns them into vampire pigs. A King Pig variant of them called Dracula King Pig appears in ABE.
They look like Normal pigs but with fangs and a dark cloak.
Whenever a pig is nearby this pig will turn into bat and fly to the pig and turn it into a vampire, increasing the difficulty of the game.
They would also fly away from danger making it even harder.
Worst of all they eat blue birds.