All the Creatures games were great, but Creatures 3 was definitely my favorite; the norns had more complex behavior, speech patterns, etc. Plus, you could splice different species together! If you ever get the chance, you should check it out (I think it's on GOG) and maybe download the Brain-in-a-Vat kit, if you're into the norns' inner workings. They really took neural processing and genetics to the next level in this game. I've never seen any other game AI come close.
As a kid, I remember my very favorite norn had a mutation that would only let her walk right; her gait genes were broken and she couldn't use left-facing sprites. To keep her alive, I had to pick her up and stick her all the way on the left side of the ship so she could eat, sleep, and play with the other norns as she slowly migrated to the right side. Then she'd get stuck against the right wall and begin to starve until I rescued her again. (I had to make heavy use of reward and punishment to train her to only walk right when she absolutely had to.)
I remember that you could send a norn with a genetic condition like this to the developers, and they'd try to diagnose what was wrong and fix it. So cool. I wonder what they're up to these days?