User blog comment:Toadstool/FAQ/@comment-24.12.214.185-20100705114326

Just to clarify on Pip...

Pip was pretty much removed from the series in Season 4 after the episode "Pip" - he was still seen in the classroom in most episodes until "D-Yikes!" in Season 11, after which he was removed completely until "201". He was also omitted from "Follow That Egg!" in Season 9.

During the commentary for "Two Guys Naked in a Hot Tub", they admit Pip was mostly cut because of Butters' growth - as they claim "Butters is a better character than Pip"