Muggles' Guide to Harry Potter/Magic/Fire Crab

Overview
Fire Crabs appear only once in the books, in Harry Potter and the Goblet of Fire, and then only in passing. They are not described in the principal books and play no real role in the story.

Extended Description
Fire Crabs are mentioned only in passing in Harry Potter and the Goblet of Fire, where a story written about Hagrid by Rita Skeeter suggested that the Blast-Ended Skrewts that he was raising looked to be unauthorized cross-breeds between Fire Crabs and Manticores.

From Fantastic Beasts and Where to Find Them, we learn that fire crabs are commonly mistaken for turtles with jewel-encrusted shells. Fire crab shells can also be used as cauldrons.