Cacophony
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Cacophony (Japanese: そうおん, Noise) is an unused ability which only exists in Generation III. The ability was removed from Generation IV and onward.
The effect of this ability is identical to Soundproof; any Pokémon with Cacophony as an ability will be immune to attacks such as Uproar and Grasswhistle, however no Pokémon can ever have Cacophony as an ability without the use of a cheating device. Furthermore, the ability is not mentioned anywhere in regular gameplay.
It is theorized that Cacophony was originally meant to be the ability of the Whismur evolutionary line instead of Soundproof, as both abilities have the same effect, and the Whismur line is noteable for being loud. It wouldn't have been the only pair of redundant abilities in the game, with Insomnia and Vital Spirit being examples. Why Soundproof was used instead is therefore a mystery.