Game Boy / Game Boy Color never had an FPS (i mean, an FPS, not shooter like Faceball from Xanath or X from Nintendo) so Slitherine thought ... why not. They develop a very preliminar orthogonal raycaster engine with good speed, good amount of sprites on screen and good story. But Eidos, their Publisher, said bye bye coz of economic problems and project never was finished. It was intended to hit the Game Boy Color market at the end of March 2000. Nintendoplayer have released the long time awaited prototype, sold on Ebay a few weeks ago, and here is. Only compatible with KiGB emulator. But it crash later in the game and when using multiplayer. http://www.nintendoplayer.com/unreleased/ The game have no sound (at least this build) so non-related (but equally awesome) Xenophobe music added.
I remember that ages ago on Slitherine forums there was a thread where they were considering T-Tex source release. Too bad it never happened.
When I saw someone mention this in that other thread I assumed it was dumped long ago and I just was unaware. NintendoPlayer is what we should all strive to be