Family Guy (partially found unaired pitch pilot episode of animated series; 1998)
Family Guy is an animated sitcom created by Seth MacFarlane that premiered on January 31, 1999. Before it's premiere, there was an unaired pilot made sometime in 1998. This pilot would eventually become the series premiere, Death Has a Shadow. Many differences from the final version can be seen throughout this pilot, such as different clothing, different character designs, and multiple scenes that were eventually cut. Aside from that, there's not much that's different from the final.
The first 7 minutes of the pilot can be found online, but the rest of the pilot is replaced with a "Coming Soon" bumper. The rest of the pilot remains lost, and the only proof of it's existence is through low-quality images from Tripod, and from a few images on the official IMDb page. How long the pilot originally was remains a mystery.