Grunt in a barrel
From Halopedia, the Halo wiki
The Grunt in a Barrel is an Easter Egg included in the Anniversary Map Pack on the multiplayer map Penance. It causes a Grunt model inside a wooden barrel to occasionally be sent over the waterfall into the water below.
Details
At the waterfall a Grunt in a wooden barrel will spawn after ten minutes or so 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" 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.
