Schoolyard Safari (2002-2003 Nickelodeon Short Series)
Schoolyard Safari is a series of 1-minute animated shorts that aired on Nickelodeon (and later Nicktoons TV) hosted by a kid who treated a regular school day as though it was a big safari.
As of 2015, only five shorts has been uploaded to the Internet, and funnyazhellanimation.com (the creators) stated that there were six produced. Some people also remember seeing this series dubbed in Portuguese, on Nickelodeon Brazil.
The reason that this particular short series hasn't been uploaded in full is likely because Nicktoons TV was not in the basic cable package at the time these shorts were aired. Therefore, it is unlikely anyone that wanted to could have recorded them. It is also likely that Nickelodeon has forgotten about the series (a similar hypothesis has been brought forward with another series of lost Nickelodeon shorts, Astrology with Squidward).
UPDATE: In December of 2015, the series was found in full, and mirrored onto YouTube by AnimatronicPony.