I was going to include a section about tardigrades, until I went to some online sources and learned something: tardigrades aren't actually considered to be extremophiles! While some CAN survive harsh environments, they typically do so by going into a stasis or suspended animations, so they are not truly "living" in those biomes. They kinda just hibernate until conditions are better, and they can be easily killed off by poor conditions.
Although, if you ask me, any animal that can survive being in space deserves the title of extremophile! :)
Thanks for the comment!