Griever

A Griever is a creature that lives in the Maze and comes out after dark. Its purpose is to kill the Gladers, with the exception of Gally.
It is described as being a bulbous, dark creature, with many appendages such as spikes, shears, and rods. A Griever can "sting" Gladers, or prick them, which causes extreme pain for up to days or weeks. Stung Gladers who take the Grief Serum are able to regain some of their memories during "The Changing." If a Glader does not take the serum, they die.
Grievers make whirring and clicking sounds when they move. The machinery inside of them often causes the reflectance of light, as described by Thomas. Grievers have stung four major characters in The Maze Runner: Gally, Alby, Ben, and Thomas. 
Grievers appear to be easily tricked, as seen when Thomas and Minho tricked the Grievers into going over the Cliff. It is unknown if the Grievers actually intended to go into the Griever Hole. They are assumed to "charge" in oblong cell-like cages seen in WICKED headquarters and at the end of The Scorch Trials.