Spent too long on this! Being a Computer Scientist though, after a little tinkering and error log checking, I found the fix.
Go to Hybrid.Animals.v1.3.1/HybridAnimals_Data/StreamingAssets
Create a new folder. Call it "_CustomCreatures" (VERY IMPORTANT - CASE SENSITIVE!!!)
After creating your creature, copy the .png and .tbxx creature files to "_Creatures", and then rename the .tbxx extension to .tbc
RESTART YOUR GAME. After that, you can go into "Experiment" and it will show up. ALSO. After creating the folder "_CustomCreatures", the load function in the creature lab WILL WORK. Have fun :) Hope people get use out of this!!!
Have fun :) Hope people get use out of this!!!