Sea Monsters (true name unknown) are giant aquatic creatures that are native to the cosmic ocean of Varuna. They have been domesticated by the resident Daagon of Varuna, who use them for transportation. The Kraang have also managed acquire a small population of these beasts and used one to guard their fallen Technodrome for a year until it rose once again.
The Kraang brought a male sea monster from Dimension X to guard their underwater base. This sea monster had later fallen in love with the Turtle Sub (which it misinterprets as a female), but was "rejected" when the Turtles (mostly Donnie) needed to go back and help April by using smoke bombs to make it release it.
The same Sea Monster appeared as one of the many things recollected in Mikey's subconscious, and winked to the camera after the turtles were cheer at their success in saving Mikey's mind.
A school of sea monsters were used by the Daagon to apprehend the turtles, April, Casey, and Fugitoid, believing them to be a threat. However, once the Daagon learned that the were affiliated with the Utrom, they allowed the heroes to ride the Sea Monsters back to the dwelling of their supreme ruler Hiidrala.
- Inside the T-Sub vehicle toy there's a small computer screen with a image of the sea monster's concept art.
- The nickname given to the monster by the production staff is "Sheldon the Sensual Sea Serpent".