Doctor Krabs (A.K.A. Dr. Eugene H. Krabs) is an alter-ego of Mr. Krabs seen in a mid-1980s Krusty Krab commercial. He states that eating Krabby Patties improve your health, though, this is not the case. His only appearance is in "Truth or Square."
He is just pretending to be a doctor so that fish could, and would eat Krabby Patties.
He was only seen once in a commercial.
He was portraying as a doctor, which, in fact, is illegal, but has been done by Patrick twice, in the episodes "Suds" and "The Splinter."