This is for having a family of rare animals.
Rare animals are denoted by having a extra noise-like colorful texture overlaying their body.
This achievement USED TO unlock during the tutorial level 'Rarities and Rewards' and it SHOULD unlock, during this level, but for whatever reason it no longer works. In addition to the tutorial, rare animals should occasionally spawn during natural game play.
I have spent a few hours trying different methods of getting rare animals in a free play tank to no avail, according to the dev, rare animals aren't actually that rare, but I have gone through thousands of generations of fish, amphibians, and other critters and have only seen a rare animal in the tutorial level, which leads me to believe something broke. There is no cheat code for spawning a rare animal either. I have tried 'iwishforfish' a few times, which spawns a shitload of fish, but no luck.
I am unsure if this game has been updated since it released on Jan 12th 2024, but if anyone has the un-updated originally released build, I think it may be possible to go offline, launch the game and complete that tutorial level, then go online and the achievement would presumably unlock.
I believe this achievement is discontinued. I would love to be proven wrong, and if you find a way to unlock this, please let me know, take screenshots if applicable.