Greenies are green rattlesnakes that developed wings after the creation of the FAYZ. They were first discovered by Lana in Gone, when one kills a coyote attacking her. They also eject drops of black liquid with streaks of bright green; this liquid leads to an infestation of bugs, who do not seem to be the same species as greenies.
In Plague, the greenies have moved from the desert into a cave by the edge of the forest, stinging anyone who goes near. Sam, Dekka and Jack try to wipe them out to stop more bug infestations after witnessing Hunter's death, but the greenies squirt the infecting liquid and two drops sting Dekka. One drop lands on her trousers but doesn't soak through and one lands in her arm but is burned out by Sam.
It is unknown what happened to the greenies after the FAYZ ended. As they were altered physically, they may have survived, or they may have died in the forest fire caused by Gaia.