From Lost Fables Wiki
It is unknown why some crabs are able to grow without end, while others are locked to their diminutive forms, though there are many theories involving the meddling of Dirgodog, demonic (or celestial) intervention, eggs laid atop ley lines, or just random chance. Perhaps it is all of these things, and they can arise in many different circumstances. Regardless of their of-times murky origins, giant crabs are much like their smaller cousins, with the sole exception that they simply never stop growing. So long as they can continue to find food to sustain their hulking form, they will continue growing, sometimes to their detriment, as the most frequent deaths they suffer is from simple malnutrition. Sometimes, however, they make nuisances of themselves when they get large enough to view man-sized targets as suitable prey, and in these cases, they must be removed by plucky adventuring sorts or the local militia. The upside is that when one of these things is killed, the whole village gets to eat crab for a night! For this reason, many fishing villages see them as good tidings.
Fueled by hunger.
Variable. They won’t attack things bigger than themselves, but a giant crab grown massive can be extremely dangerous.
Tough shell, very strong, can eat anything.
Somewhat slow, extremely dumb. Will eat anything.