Moai statues are a type of structure seen on Easter Island. They are huge rock structures carved to vaguely resemble human faces. Although they exist in real life, they make cameos in Konami games as well.
Relation to the Metal Gear series
Because the Metal Gear franchise was produced by Konami, a Moai statue makes a frequent cameo in some of the games:
In Metal Gear Solid, the Moai statue can be found in basement 2 of the warhead storage building.
In the Déjà Vu mission for the PS3/PS4 versions of Metal Gear Solid V: Ground Zeroes (pre-May 1, 2014), the player has to find a Moai statue in the admin building's basement to recreate a scene from Metal Gear Solid. Upon doing so, Big Boss/Classic Snake will be heard saying "I can't remember..." In both the PS3/PS4 and Xbox 360/Xbox One versions of the same game, a trial for Intel Operative Rescue on Hard mode has the player shooting out Moai statues littered across the base while in the chopper.
Although not directly related to the Metal Gear franchise, the multi-party fighting crossover game DreamMix TV World Fighters secret character Moai was based on the Moai statues that appear in various Konami games as secrets, including the Metal Gear series (in particular Metal Gear Solid and Metal Gear Solid 2, the games present at the time the game was released).