Truffle

The  is a hard mode NPC and will only show up once you have constructed a house in an above-ground mushroom biome with Mushroom Grass Seeds, or by using the Clentaminator in a Jungle. If you want to do this but don't want to take up a lot of surface space, a good way to do it is to build a skybridge of mud off of a Floating island, plant Mushroom Grass Seeds, and then wait. (Note that the skybridge must be 2 mud blocks thick, and that the house must meet all of the other normal NPC house requirements.)