The Sea Monster is a large, quadrupedal monster in the TMNT universe. Like the rest of the Thirteen Monsters, the Sea Monster escaped from a portal in South America 3000 years ago, and has been terrorizing humanity ever since. This monster inhabits the water, where many legends about the creature have developed.
In the 2007 film, it is the final monster needed to open the Stars of Kikin portal. While the Turtles hold off the Stone Generals (who wished to keep their immortality rather than have it revoked by the portal), April, Casey, Karai, and a Foot Ninja head off to lure it back to Winters Towers.
In the GameBoy Advance version of TMNT, the Sea Monster (with a much friendlier appearance) functions as a pseudo-boss. Players must feed it 20 giant fish to "defeat" it while battling Purple Dragons.