You will need to have unlocked an animal actor by collecting a Peyote Plant before you can unlock this, heres a list of them
http://gta.wikia.com/wiki/Peyote_Plants see the following solution for more details.
Solution for Cryptozoologist In Grand Theft Auto V (Xbox One)Whilst story mode is loaded up press

and scroll right using

to the 'Rockstar Editor' menu and press

to enter. Choose 'Director Mode' from here and accept the launch.
Click actors and pick an animal from the animals menu.
Once the mode loads the achievement will pop, here's a video