Canceled for being too extreme
The titled epitome of ’90s raditude done in by hubris, Sonic X-Treme, was meant to be the first full 3D Sonic — a Mario 64 colleague — and the first of SEGA’s star series for its Saturn system. Like so many subsequent 3D Sonic games should have been, Sonic X-Treme was cancelled. The Saturn lacked its company mascot, and ultimately SEGA was done in on the hardware side.
We’ve seen leaked footage of Sonic X-Treme before, but dedicated fans have actually put together a playable demo, which you can get here with a forum registration.
As the thread notes, it’s just one level — with “more builds and levels to come later” — that is “an almost as is port of the engine…how the original developers had left it.” The fish-eye lens that characterized the canceled platformer will come in a later version.