There has been a lot of talk over the years of having an in-game bestiary. Many people want it, and Crate has even talked about the challenges of adding one. Please reconsider adding one. 
It would be amazing to have a Bestiary, and with the new/final expansion coming out, it’s probably the last time to implement such a feature.
Here is a concept I made to see how it could be: Top left buttons toggle between Faction list and Bestiary list.
The art/description used is from Allminoxy’s first bestiary post from September 2012.