Pigron

Pigron is a Hardmode monster found in the Underground Snow. They have a special ability to turn "invisible". If you take a closer look you will see a silhouette flying through the air. As they start to attack you, they will become visible again. If you're trying to escape him, he will follow you till his death. In the invisible state they can fly through walls.

Trivia
The name is a combination of the words "Pig" and "Dragon". This was confirmed by Cenx in the IRC.

They actually look very similar to Pommy Dragon from the game "Bomberman Tournament", it is unknown if they are related or not.