Before X-Box, before Halo:CE there was a Halo game that was orginally announced for the Mac as a Myth-style strategy game[1]. The development time was from around 1998 to the release of Halo: CE. AS the game evolved along the way some vehicles and weapons were cut and others had complete overhauls. Below are some examples of lost or heavily modified game play items:

Gattling Gun

File:Halo original 4.jpg
Pre-Xbox render

The Gattling Gun was a weapon originally designed for the Pre-Xbox Halo Combat Evolved. It seemed to be a hand-held version of the M41 LAAG, but, as it was not put into the game, it's characteristics are unknown.

There have been several mods, using Halo Custom Edition, that have integrated the Gattling Gun into the game, using the handle and base of the Flamethrower, but replacing the adhesive projector with the barrel of an M41 LAAG.

Machette

File:Machette vs EnergySword .jpg
Machette vs EnergySword

A Machette was at one point part of the Halo game play. Also notice how much Master Chief has evolved.

Elites

An Elite circa 1999. They've come a long way!

Spectre

The Spectre was originally designed to be in Halo Combat Evolved, but was cut because of balance issues. There appears to be a Shade turret mounted on the back.

Old HUD

File:Oldhalo.jpg
The old HUD Display and a Halo that has fallen upon disrepair

The Heads Up Display didn't always look like its present form. Shown here is what it used to look like. Also notice how the Halo in the background had orginally fallen into disrepair.

Beam Rifle

File:Halo edge 3.jpg
The Halo 1 Beam Rifle

The Beam Rifle was another element that was deleted from Halo Combat Evolved. It was cut, once again, for balance issues, but it vaguely resembles Halo 2's Beam Rifle, here seen brandished by an Elite.

Sources