Mr. krabs is a cooked coconut crab
here's why:
Krabs has insanely large claws for a regular crab. He also has eyestalks which are not natural in true crabs, which is what Krabs is supposed to be. But then look at a coconut crab. It has huge claws, a similar shaped body, and eyestalks. Coconut crabs also have a protrusion in the front of there head, like Mr. Krabs's nose. To explain the cooked part, here's a sentence from Google:
"Shells are red and blue in color but turn bright red when cooked"
Also as a little joke part of the theory, he got cooked because Squidward got very angry and threw Mr. Krabs into the fryer, but he escaped. This all happened before Spongebob came, because when Spongebob sees Krabs he's already bright red.