A Youtube Poop (YTP) is a remix of a video into something non-sequitur and humorous. (That's my best shot at a definition, anyway.) The appeal of poops for me has always been strictly not in defaming the original video, but in using it as a base for audio-visual jokes that couldn't have been made on their own.
From my experience, Youtube Poops were most popular around 2008-ish. Some time later, German-language Youtube Poops ("Youtube-Kacke"; YTK) started to catch on, peaking in popularity around 2010-11. I was rather active in that "scene" around the time and uploaded a handful of German Poops, some of which enjoyed moderate success. I still put poops out there occasionally, but it feels like there's really not much of an audience left for them.