I found differing info on a googling, so is it or is it not?
And after that want is the largest animal that could be thus classified?
The term extremophile was originally coined to refer to microbes such as bacteria and archaea. It doesn’t have any formal definition, so there’s no real answer to your question.
I don’t think that most biologists would use it in reference to multicellular life (although the Wiki article on the term does), and certainly not to vertebrates like penguins.
Agreed. I’ve never heard the term used to describe anything except microbes.
I’ve heard it used to describe tardigrades or “waterbears”, which are multicelluar although whether they’re actually extremophiles is very much arguable as well. Another possible multicelluar extremophile candidate is the brine shrimp, which lives in salty environments, and some of the critters living around undersea hot vents. Again, though, there seems to be an unspoken assumption the term only applies to single-cell organisms - or maybe there is such a formal definition that I’m simply unaware of.
I dunno, if we ever found, say, a worm that could live in Yellowstone’s hot springs, I would feel comfortable calling that worm an extremophile. It’s just that we haven’t found any such worms.
Emperor penguins certainly live and thrive in an environment significantly different from what humans do, but I’m not sure if I would quite call it “extreme”.