The human bat (also called the Bat Thing) was either a human that had evolved into a bat-like creature, or vice versa. Described as "neither man nor animal", it was active only at night and hung upside down from rafters during the day, leaving distinctive claw marks in the wood.
Discovery by Dana Scully and John Doggett
In 2000, a burnt corpse was fished out of a river in Idaho, and was later identified as Ernie Stefaniuk's wife. Coinciding with the discovery of the body, the human bat reappeared and began a new spree of killings. It first killed a mortuary worker and his wife in their rural home, both of whom had prior contact with the burnt body. It then entered another home closer to town, were it savaged an elderly woman in her attic. She was later identified as the mother of Stefaniuk's dead wife, who had identified her daughter's body for the local PD, thus having made contact with the body.
At this stage, it became obvious to Agents Scully and Doggett that the creature was killing anyone who had established a connection with the corpse, which harbored the scent of Ernie Stefaniuk, and it was presumed that the creature was seeking revenge for its capture a half-century ago. The agents located Myron Stefaniuk, the brother of Ernie, and asked him where Ernie was. They later found Ernie, who had been hiding in a cabin on a secluded island for forty years and warned the agents that, even though they had seen that Myron was unharmed during the day, the creature attacked only at night. Doggett went out to the boat in an attempt to reach Myron, who lived on the shore, but was attacked by the creature and mauled unconscious. A loud banging soon started on the roof of Ernie's cabin, and Scully shot at the ceiling several times and then went out to investigate. The human bat then quietly entered the house and murdered Ernie. Scully shot it at least four times, and then Agent Doggett returned, barely conscious, and shot it three more times, but it flew away into the night, screeching. Its ultimate fate remains unknown. (TXF: "Patience")
The role was played by stuntman Jay Caputo.