Grunt in a barrel
From Halopedia, the Halo wiki
At the waterfall, a Grunt in a wooden barrel will spawn after a random period of time and fall over the waterfall. If it lands in the water, it will sink and disappear, but one can create a structure on Forge to catch the Grunt.
The Grunt itself has no mask, no methane pack, and does not move or respond. It is in its default "T-pose" wearing untextured armor (causing it to appear as solid black) and is standing inside a wooden barrel. The Easter egg as a whole is a reference to adventurers who had ridden over Niagara Falls in a barrel, such as Annie Edson Taylor, or Bobby Leach.
This Easter egg was featured in Rooster Teeth's Top 10 Easter Eggs of 2011. This Easter egg specifically made the list at #4.