Source: RipTen Videogame Blog
Find and kill a chupacabra.
This is the last challenge on the Undead Hunter Challenge
The greatest chance of finding the Chupacabra is in Nuevo Paraiso when you are on the last tier of the hunting challenges. When you are near the random Chupacabra spawn, a message will appear and you will be able to see the radius of its possible location on the map.
* The Chupacabra can be found in the fields North West of Torquemada. When zoomed out on the game map, the area will have a picture of three horses.
* The creature can also be found South East of Perdido
* South of Casa Madrugada, or North of Chuparosa near the river.
Video courtesy of Yumiya89
Thanks to STLfan2468: If you can't find the Chupacabra, save your game at Chuparosa and QUIT the game. Load the game up and head to the area south west of Las Hermanas. The game will prompt you that a mythical creature is in the area.