|The following is based on Van Buren and has not been confirmed by canon sources.|
Two-headed Rattlesnakes are creatures found in Mesa Verde in 2253.
They're the pets of the Vipers camped in the canyon. The Vipers give the snakes freedom to roam, and the snakes are usually found in the underground caverns hunting mole rats. The snakes will be randomly roaming the Mesa Verde caves. If they encounter a mole rat, they'll attack it.
Two-headed Rattlesnakes were to appear in Van Buren, the canceled Fallout 3 by Black Isle Studios.