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, trying to eat him. They were very dangerous, and ate everything they could, such as bricks and ? Blocks. There was no way to defeat a Mega Unagi. However, Mario could go through one with a Mega Mushroom.

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".