Haunter

Despite Haunter being inferior to its evolution, Gengar, in every meaningful way, it holds the smallest of niches thanks to their shared Ghost typing. Haunter can switch into Explosion freely and wall attackers that only use Normal-type attacks. Gengar is better at this, but having Haunter as a backup lets players use Gengar more aggressively and bait and punish an Explosion after Gengar faints. However, most of the time teams don't want two Pokemon that perform essentially the same role and vastly prefer just keeping Gengar around. Additionally, Haunter is very underpowered for STABmons; it struggles to deal meaningful damage to the metagame's plethora of Normal- and Psychic-types, leaving it as a gimmick.