Post Pokemon that had a rags-to-riches-to-rags story (was shit when it came out, became pretty good later, then reverted to being a shitmon) in competitive (Smogon and/or VGC) Politoed is the first thing I could think of, he was a shitty generic bulky water when he came out, was the most defining and important mon in gen 5, and reverted to being shit upon the removal of permaweather.