A. Do you mean extremophiles? No fossil evidence supports large extremophiles*I would wager that those creatures would have been bigger in earlier time periods on earth, and if we’re talking fantasy beasts then all bets are off.
B. *sigh* we're talking here as if they are real, flesh-and-blood animals and comming up with theories on how they could function and how they could exist
*excluding deep-sea animals, as, for some ungodly reason, organisms get really big and freaky the deeper you go, kinda, said ungodly reason is because larger organisms have a more efficient metabolism