Mega Unagi

Mega Unagi are a gigantic version of Unagi that appear only in New Super Mario Bros. Mega Unagi appear in two underwater stages in the game, 4-3 and 8-3, the latter in which one chases Mario (or Luigi) throughout the latter half of the stage. They are very dangerous, and destroy everything they can, such as bricks and ? Blocks. There is no way to defeat a Mega Unagi. However, Mario can go "through" one with a Mega Mushroom or a Starman, by swimming through its back, as Mario will collide with it if he tries to go through its face. However, in World 8-3, when Mario moves alongside its face, Mega Unagi will swim slower and also try to crouch down on and corner Mario to the top or bottom surface of the screen. As long as Mega Unagi is positioned at the very top or bottom of the screen, if the player crouches down at the very bottom of the ground or stays at the very top edge of the screen while Mega Unagi tries to crush Mario with its body, eventually it will swim forward and vanish from the screen, but will return after approximately one minute. Unlike Cheep-Chomps, Mega Unagi cannot eat Mario and cause him to lose a life instantly; instead, the player will take damage like a regular enemy.

Normal Unagi are larger in most subsequent appearances and thus appear to be closer in size to Mega Unagi.