A mythical creature used in a practical joke by salty Marines and sailors against inexperienced compatriots, usually aboard ship. There are a number of permutations of this joke, some quite nasty. One is that a newbie is shown a helmet laying on the deck and is told that someone has captured a sea bat. He is then told to position himself with one hand on each side of the helmet so that when the helmet is lifted he can capture the bat with his hands. The “bat” turns out to be a pile of feces or some other nasty substance.