Sabrina's Haunter (Japanese: ナツメのゴーストNatsume's Ghost) was a Pokémon briefly in Ash's possession. Because Ash didn't formally capture this Pokémon in a Poké Ball, many fans don't consider it to be his Pokémon.
Ash decides to challenge the Gym Leader, Sabrina in Saffron City, unaware of what happened to many of the challengers that lost to her. A man tries to dissuade him from challenging her, but Ash does not listen, intent on getting another badge.
Sabrina uses her psychic powers to turn Ash and his friends into miniature versions of themselves before they are rescued by the man who warned them about Sabrina's powers earlier and turns out to be her father. Sabrina's psychic powers far outclass her father's, leading him to have no control over her anymore. She had turned into two people - her old, fun-loving self and a cold-hearted Trainer with a thirst for power. Her evil self had locked away her soul.
Sabrina's father tells Ash that Ghost Pokémon would probably be able to defeat her Psychic minions and suggests going to the Pokémon Tower to find Gastly, Haunter and Gengar. Ash finds all three of them, but does not capture them. A Haunter that he befriended follows him back to Saffron, but when Ash is ready to fight, it disappears.
When Ash finds it, it disappears once again. Regardless, Pikachu asks to battle Sabrina, but again, Ash's defeat is imminent. However, Haunter suddenly returns, making faces at Sabrina. Sabrina finally laughs for the first time in years, making the connection to her old self again. She is immobilized by laughter at the Haunter's faces, and her Kadabra, who is psychically linked with her, is immobilized as well and Ash and Haunter win the Marsh Badge.
Haunter decided to stay behind with Sabrina at the Saffron Gym.
Haunter is the second Pokémon in the anime to be under the ownership of a Gym Leader that is not of their specialty type, after Brock's Zubat.
Haunter is Ash's first unofficial Pokémon. Despite this, however, Sabrina and Ash both stated that Ash caught Haunter in Haunter Versus Kadabra (implying that it was official).
Haunter was the first Pokémon Ash has owned that was in the Indeterminate egg group.
Haunter is Ash's only Pokémon, together with Beedrill, to have only one of it's attacks revealed, in the case, Lick.
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For more information on this Pokémon's species, see Haunter.