Sushi

Sushis are a shark-like enemy found in several Mario games.

Super Mario 64
Sushis first appeared in Super Mario 64, where they could be found in Dire, Dire Docks. These sharks swim around, minding their own business. There is no way to attack them, and they usually won't bother Mario, unless he accidentally swims into them. They also reappear in the DS remake, Super Mario 64 DS, acting the same way as they did in the original.

New Super Mario Bros.
Multiple Sushis also appear in World 1-A in New Super Mario Bros.. They send 2 at a time. One slightly above Mario and one under.

Mario Party
In the Mario Party mini-game Treasure Divers, a Sushi appears swimming around alongside two Bloopers.

Mario Party 2
In Mario Party 2, one appears in Pirate Land, and will ferry anyone who lands next to the dock of one island to the dock on the opposing island, charging five coins per trip.

Mario Party 3
In Mario Party 3, a Sushi appears in Deep Bloober Sea, and is in charge of the torpedo controls. This Sushi will ask everyone who passes to push one of four buttons upon reaching a fork; one of which serves as the detonator, which will fire a torpedo when pushed, forcing the player to take the path they didn't select.

Mario Party Advance
In Mario Party Advance, a Sushi residing in Sushi Cliff appears asking the player for help after Shroomlock was pulled into the sea.

Mario Party 7
In Mario Party 7, the 1-vs-3 mini-game Be My Chum! involved the lone player calling out names of various enemies (including Sushi) on the Nintendo GameCube Microphone to attack the other players. They simply swam straight forward in a speedy manner.

Trivia

 * Sushis got their name from sushi, a type of Japanese food that can include fish.