The original Creatures I found in the going out of business sale at KB Toys when I was a kid for like $2. IIRC, it was also the first game I ever bought on my own.
To this day, it's still be the best virtual pet game I've ever played. The whole system of genetics was cool as hell, and the fact it was made by scientists studying the human genome and uses real science behind the genetic modeling was super fucking cool.
I loved that game even though I never figured it out. I couldn’t find anything else like it. It’s one of the reasons Spore disappointed me so much. Spore had customizable appearances but all the creatures acted the same. In Creatures, it almost felt like they were alive.
The memory I have ingrained most from the game is that once I hatched a norn that for whatever reason, could not walk. It just stayed where it hatched unless another creature moved them. I feared it would starve to death, but the other norns took care of her. They brought her food and liquid, they played with her, they taught her language, they even protected her from the Grendel. This wasn't necessarily pre-programmed behavior; this never happened at any other time for me. They usually didn't teach each other much, and they certainly never brought each other food or gifts. Just for the one that was unable to take care of itself.