Ash's Pikipewpew is a one of a kind Gun/Flying type Pokemon. It was first captured on Episode 7: Grand Theft Pikipek as a Pikipek and was then used as a weapon. It was not seen again until Episode 40: Return of the Summaries, where it immediately evolved into Pikipewpew. Since then it has gone back and forth between life and death, and is the first non-fully evolved Pokemon to successfully fuse with Ash.
Pikipewpew
It evolved into Pikigatling on Episode 163, yet soon reverted to it's previous form for unknown reasons. It once again evolved into a Pikigatling on Episode 223: I don't even, except it's evolution was maintained. In Episode 225: Dave Vs. Ned it showed that it is still able to merge with Ash. Ash-Pikigatling was able to KO Dale after he had shot Hank Hill.