Simple

When one of this Pokemon's stat stages is raised or lowered, the amount is doubled.

5 Pokémon have this ability

#322

Numel

FireGroundHoenn

#399

Bidoof

NormalSinnoh

#400

Bibarel

NormalWaterSinnoh

#527

Woobat

PsychicFlyingUnova

#528

Swoobat

PsychicFlyingUnova