From the Super Mario Wiki
Mega Unagis are a gigantic sub-species of the Unagi that appear in New Super Mario Bros.. Mega Unagis chase Mario (or Luigi) through two underwater stages in the game, being 4-3 and 8-3, trying to eat him. They are very dangerous, and eat everything they can, such as Brick Blocks and ? Blocks. There is no way to defeat a Mega Unagi. However, Mario can go "through" one with a Mega Mushroom or a Starman.
In Mario Kart Wii, Mega Unagis appear in the Chain Chomp Wheel battle course and in Koopa Cape as freely-swimming eels that swim under the tunnel, unlike their smaller relatives in the same course, who are seen as living in pipes.