Poof has appeared in every episode since "Fairly OddBaby" in 2008 (his debut) which was shown at the start of the series' sixth season. This makes him one of the few characters to appear in nearly every episode since he was introduced. He appears in the movie as well.
This episode marks the birth and debut of Poof though his gender and name were not official until the end of the movie. As such, there is a running gag with characters calling Poof a different name or people trying to find out what gender he was.