After you have completed Undead Hunter Rank 4 in Red Dead Redemption Undead Nightmare, you need to kill Chupacabra to rank up to legendary. You can refer to our Undead Nightmare Level Up Guide for more info.
Now, the probable locations to find Chupacabra are South of Perdido, South of Sepulcro, Casa Madrugada, and Mesa del Sol, all of which are in Mexico. When you have come near to this mystical creature, a royal blue circle will notify you of its presence in the area.
You can refer to this video guide below on how to locate and kill this creature and rank up as Undead Hunter. You will also unlock the trophy Chupathingy when you kill this creature.
Now follow this video guide for the exact location of the unicorn and unlock Fan Service trophy.