The One Who Lurks in the Deep Sea, colloquially known as the Fishman is a character briefly introduced after the credits sequence of Metal Slug 3. When Player 1's rescuer tosses their pistol into the ocean, the weapon sinks into the deep sea until it lands on the fishman's head.
Said character would return in Metal Slug Attack where he's a playable unit as part of the game's April Fool's Step-Up Crank. His appearance is the same as Metal Slug 3 except his body has been enlarged (his head is the same size as the Huge Hermit). He serves as moving wall with no offensive capabilities. All four of its skills increase its total HP. Each frame costs 150 medals after spinning the crank four times for free.
- The gun that sinks to the ocean depends on the character who threw it. The .50 for Marco and Tarma and the .38 for Eri and Fio.
- The Ones Who Lurk in the Deep Sea would've appeared in Metal Slug 4 (created by the original staff) as either NPCs or enemies in an underwater city mission. The fishman's brief appearance in Metal Slug 3 was a hint for the sequel. This was scrapped due to the team being unable to put the concept together, a lack of time, and SNK's financial problems in 2000.