Earliest Primus demo I know of. Recorded with a drum machine because the the band lacked a drummer at the time. It sounds like a wierd new-wave version of Primus. Highlight is definitely Too Many Puppies.
There's a band called PH8 from Tucson, Arizona that had demo versions of their songs Breaking the Tension and Plainside used in promotional POSTAL 2 videos. I can't find demo versions of these songs specifically.
There's another band called Innisfail and the only proof of their existence is an archive of their website and one of their songs being in a POSTAL 2 video.
Walk on Water by Ozzy Osbourne. This is the version that appeared in Beavis and Butt-Head Do America, and is the demo. It is widely considered superior to the official release, even by Ozzy himself.
The official version:
Between 1994 to 1997, when Pete Lee was their guitarist, GWAR did a lot of demos that were halfway them just fucking around. A lot of it was compiled into what's known as Slaves Going Single, and existed through the Slavepit fanclub for a long time. They eventually sold it at merch tables at shows. Now, it all exists as a playlist on their Youtube page. A couple of songs got reworked or found their way onto official albums. A snippet of Drop Drawers was snuck onto the album Carnival of Chaos after the song Don't Need a Man, as a hidden track.
The Needle, with Mike Derks AKA Balsac the Jaws O Death on vocals, got reworked into a song from We Kill Everything called Escape from the Mooselodge: