New Slimes can be found by opening Slime Gates with Slime Keys, which you get from feeding the huge Gordos.
I downloaded this without knowing if I would like it or not - I was thinking a bit about how Bugsnax was a big disappointment, and this game occupies a similar space in my head. But Slime Rancher is super fun, charming, and satisfying.
This is a game about harvesting and selling poop from slimes.
I had captured a bunch of different slimes, but were unable to feed them properly. Too hungry, they started breaking out of their cages and ate each other's poop, which turns them into huge uncontrollable hybrids. The hybris were friendly, but I couldn't contain them or feed them, and the whole farm seemed impossible to salvage. But then something interesting happened: the slimes turned into Tarr Slimes, which started attacking and eating my other slimes. After I had blasted all the Tarrs out of my ranch, all my slimes were gone. Now I had the perfect opportunity to start over and create the perfect balance of crops and slimes. As I understand it, the Tarrs will eventually appear when the big slimes eat each other's poop, and will force you to start over and create something more balanced. This seems like an intentional design choice, very clever.