Welcome to Red Dead Answers. What would you like to know?

First achieve up to rank 4 of Undead Hunter or something like that, then kill the chupacabra and go to the place in Mexico to the far right of the map. (Picture of 3 horses near it) You will find it there. Oh, and you have to be playing undead nightmare.

It's rank 5 of Undead Hunter. Find and kill the Chupacabra and then go up to the aforesaid place (Diez Coronas, little noth-west of Torquemada). Then just ride up and down that area. The unicorn is white and trails rainbows when it runs, so it's noticeable. If you don't see it right off, leave and go ride around a ways and circle back. Keep trying, it took me an hour.