Crabs are mobs that can be found on beaches, in the Overworld. Although it is a neutral mob, it is not recommended to take on one of these mobs early on in the game. Without Rupee Armor it can kill you in two hits as it deals 13 damage (6.5 hearts) and has lots of health. In the later game it is more important to kill them because you can craft some weapons with the Crab Claw that it drops.
The player is advised to be prepared by having an enchanted bow or better. The best way to attack it is at the ground on where it stands.
There is a stonger version, called the King Crab.