You can tame the King Crab by using a Saddle with a 2% chance, which will increase when wearing a Crown of Luck to 10%. It can travel in water, although it is roughly the speed of swimming with a diving mask. It can travel in the forest, but it is roughly the same as normal walking speed. It is very slow in the winter, desert, and lava biome. Because of this, the primary reason to tame a king crab is if you have a Crab Spear and Crab Helmet: if you wear a crab helmet and ride a king crab, the crab spear deals 24 damage (equivalent to a lava spear).
Don't kill the king crab, it is easier to kill several normal crabs. If you do want to kill a king crab, do not simply attack with a sword, you will always die unless you have a mount or boat.
The easiest way to kill a king crab is to trap it in a 3x3 grid of walls and doors. Another way is to kill it with a pickaxe or machete.
The King Crab was added in the "Animals Master" update.
It's one of the fastest ways to travel in the land of the forest biome.