The Black Snail is a stray black snail who only appears in the episode "One Krabs Trash."
046b - One Krabs Trash 467
He is the equivalent to a black cat. He was shown in the graveyard Mr. Krabs went to. It is seen when Mr. Krabs had tripped and saw the black snail which made the snail hiss, and slither away. When he hissed his shell didn't seem to be hard at all like how a snail's shell is supposed to be.