From Halopedia, the Halo wiki

A fan-made image of Mister Chief created for use in Template:Realworld.
This article contains information about a non-canonical subject that, while official, is not part of the established Halo universe.
A render by Bungie employee Paul Russel.

The Threeleg was an ambient lifeform intended to inhabit Installation 04 in Halo: Combat Evolved. As its name suggests, it was a tripodal organism, with three powerful legs and two smaller arms. It appears to have had an exoskeleton, and at least two eyes. It was cut from the final game.[1]