The Body-Snatchers, additionally called Snatchers and Bioroids, are a race of intelligent, paranormal entities who possess certain personnel's' bodies and lives among their peers, appearing only in the Xbox version of Metal Gear Solid: Ground Zeroes.
Note: The following information is non-canon to the official Metal Gear franchise
In the mid 1970s, a group of body snatchers infested a US Naval facility in Cuba and possessed almost all stationed personnel. This drew the attention of private military Militaires Sans Frontières and their main operatives, Big Boss and Kazuhria Miller. As such, they sent in a cyborg from the future named Raiden (who is a primary protagonist of the chronologically later Metal Gear games) to neutralize them all as he was the ideal operative to perform such a mission since he was immune to the body snatchers possession, as they could only take control of biological beings and Raiden was mostly robot.