Several Unagi's in New Super Mario Bros.
Unagi are a fairly large species of eel found in New Super Mario Bros., plus a couple other minor appearances. Their goal in New Super Mario Bros. is to try and hit Mario by swimming into him or biting him. The ones that swim just simply swim across the screen at narrow points, the biting ones are stationary and lunge out at Mario from the weeds when he gets close.
A few larger Unagis make a minor appearance in the Mario Kart course Koopa Cape in Mario Kart Wii. During a section of the course that consists of players driving through a transparent under water tunnel a few Uagis maybe Mega Unagis are seen poking their heads out of Warp Pipes. This course is coming back as a retro course for Mario Kart 7, so it is thought they will appear in this game at the same area.