Mega Unagi

Mega Unagis are a gigantic sub-species of the Unagi that appeared in New Super Mario Bros.. Mega Unagis chased Mario (or Luigi) through two underwater stages in the game (levels 4-3 and 8-3), trying to eat him. They were very dangerous, and ate everything they could, such as Brick Blocks and ? Blocks. There is no way to defeat a Mega Unagi. However, Mario could go "through" one with a Mega Mushroom or a Starman. In Mario Kart Wii, a Mega Unagi appears in the Chain Chomp Roulette battle course.

Trivia

 * An issue of Nintendo Power voted the Mega Unagi to be the scariest villain in New Super Mario Bros., and was then called a "giant screen-taking eel".
 * in a level in world 8 on new super mario bros, there are small versions of the eel that swims back and forth. they don't chace mario unlike the big ones.