The Black Snail is a sea snail who appears in the episode "One Krabs Trash."
It is a black sea snail with a blue swirl on its shell similar to Gary's. It has a blue body with a dark purple underside and red-orange eyes.
In "One Krabs Trash," Mr. Krabs goes to Floater's Cemetery to find the number one hat. While there, he sees things like a clam with orange eyes similar to an owl, and clams like a group of bats flying past him. Mr. Krabs then comes across this black snail who scares him with his growl and slithers away.
Since snails are the equivalent of cats in Bikini Bottom, the Black Snail could possibly be a reference to a black cat, which typically is portrayed with similar behavior.