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
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