It's one of the two Kecleon brothers.
It only appears in Pokemon Mystery dungeon and the anime.
>Two Kecleon—one green, one purple—known as the Kecleon Brothers, appeared in Pokémon Mystery Dungeon: Team Go-Getters out of the Gate!
>They made another appearance in Pokémon Mystery Dungeon: Explorers of Time & Darkness.
Angela used a Kecleon to train her team of ninjas in From Cradle to Save.
It also appears in episode 205 of the Pokemon anime.
>The women explain that their Kecleon, Greeny and Reddy, (Greeny is a green Kecleon, Reddy is a purple Kecleon) are rather quite odd.
As you already know, it's not a shiny Kecleon, but it's colored differently. It isn't explained why that is, though, so I can't really tell you that.
>The purple Kecleon showed in this episode is not a normally colored Pokémon. However, its color is also not that of a Shiny Pokémon.
Sources: Kecleon bulbapedia page and the link above
Hope I helped.