As the others have said, the "Chupathingy" achievement is for killing the Chupacabra. The Chupacabra won't even spawn until you've unlocked rank 5 of the Undead Hunter challenge. I personally found the Chupacabra North West of Torquemada in Mexico. As with all undead, just shoot it in the head and it's game over :)
This video also shows me unlocking the "Mad Marston: The Trail Warrior" achievement (for completing all of the challenges), along with the "Fan Service" achievement (for breaking/taming the Unicorn). The Unicorn spawned in the same area as the Chupacabra within seconds of it dieing, as I'd already acquired all 4 horses of the apocalypse.
Hope this helps you guys :)