Animals and cash
Animals earn cash, can be upgraded, need base space, and keep earning while you are offline.
What this page will not invent
Species names, cash per second, mutation labels, and a sell price were not in the official description. Read those numbers on the animal in your server. The free animal from liking the game and joining the group is covered on the free-animal page.
Cash is also the reason to look at rebirth and the X2 Money pass.