Generation Iii
Illuminate
Encounter rate increases.
Detailed Effect
Overworld: If the lead Pokémon has this ability, the wild encounter rate is doubled. This ability has no effect in battle.
Pokémon With Illuminate
8 canonical Pokémon forms can have this Ability.

Staryu 
Starmie
Chinchou 
Lanturn
Volbeat
Watchog
Morelull
Shiinotic