Pigron

The Pigron is a hardmode monster found in the Underground Snow biome when it is overlapped by Underground Hallow, Underground Corruption, or Underground Crimson. It comes in multiple colors (depending on which spreading biome it's in) and has a special ability to turn "invisible," allowing it to fly through solid blocks. It does not truly become invisible, only highly transparent, and can be seen with close attention. As it starts to attack, it will become visible again.

Trivia

 * The name is a combination of the words "Pig" and "Dragon". This was confirmed by Cenx in the IRC.
 * Pigron looks very similar to Pommy Dragon from the game "Bomberman Tournament"; it is unknown whether they are related. It also looks similar to the Chimera enemy from the game Earthbound.
 * The Truffle seems to mention them in its dialogue.