If a Haunter beckons you while it is floating in darkness, don't approach it. This Pokémon will try to lick you with its tongue and steal your life away.
(From Bulbapedia) Haunter is a purple Pokémon with a gaseous body. It has a round head with a pair of disembodied hands. Each hand has three pointed fingers. Its head is flanked by three large, spiked protrusions on either side, and its eyes are large and triangular with small pupils. The gases composing its head curve down to form several pointed teeth framing a pink mouth and a long tongue. Its body tapers into a jagged tail.
Haunter hides in very dark places, where it plots to take the life of the next living thing that comes near it. It silently stalks its victim and steals its life force with a lick from its gaseous tongue, which inflicts convulsions that persists until the victim dies. It has the ability to float through solid walls. Despite its predatory nature, it has been known to entertain and prank people in the anime. Additionally, it has even been shown to feel intense loyalty to a deceased Trainer. Haunter lives in dark, abandoned caves.
If Foresight or Odor Sleuth is used by an opponent, or the opponent has Scrappy, the effectiveness of Normal-type moves is 1× and the effectiveness of Fighting-type moves is ½×.
If Gravity is in effect, this Pokémon is affected by Smack Down or Ingrain, the opponent has the Ability Mold Breaker, or this Pokémon's Ability is affected by Gastro Acid or is replaced, the effectiveness of Ground-type moves is 2×.
(From Bulbapedia)Haunter is possibly based on the dila, a spirit that passes through walls then licks certain humans to death according to Filipino mythology. It also appears to be based on a generic sort of cartoonish ghost.
(From Bulbapedia) Haunter is derived from haunt, as ghosts are known to do. It could also be a corruption of "hunter".