Rivalry

 is a Pokémon ability introduced in the fourth generation. It changes power of moves based on the gender of the Pokémon.

Description
Pokémon with Rivalry increase the power of attacks by 25% if the opposing Pokémon is of the same gender. However, the power of attacks decrease by 25% if the opposing Pokémon has the opposite gender. Moves against genderless Pokémon are the same attack power. Prior to the fifth generation, Rivalry powered up confusion damage.

Pokémon with Ability

 * Nidoran♀
 * Nidorina
 * Nidoqueen
 * Nidoran♂
 * Nidorino
 * Nidoking
 * Shinx
 * Luxio
 * Luxray
 * Axew
 * Fraxure
 * Haxorus
 * Litleo
 * Pyroar

As a Hidden Ability

 * Beautifly
 * Pidove
 * Tranquill
 * Unfezant