Eep Cheep

Eep-Cheeps are a sub-species of Cheep-Cheeps first found in New Super Mario Bros. Wii. They are similar in appearance to Cheep-Cheeps, although they are golden in colour and will swim away from Mario when he gets close. Their name is a combination of the species name "Cheep-Cheep" and "eep" – a short scream of discomfort when frightened easily in reference to the Eep-Cheeps nature of swimming away from Mario and living in schools for protection.

New Super Mario Bros. Wii
Eep-Cheeps make their first appearance in New Super Mario Bros. Wii. In this game, however, they were referred to as Golden Cheep-Cheeps. They appear in groups, called Golden Cheep-Cheep Schools. They'll swim away from the player whenever they get close, the opposite behavior of Deep-Cheeps.

New Super Mario Bros. U
Eep-Cheeps return in New Super Mario Bros. U. They retain the same behaviour and appearance. The fish was renamed Eep-Cheep and the group was renamed Eep-Cheep schools.

Eep-Cheeps also appear in the DLC New Super Luigi U, which also includes larger versions of them.

Trivia

 * Though first seen in New Super Mario Bros. Wii, the concept of golden-colored Cheep-Cheeps was originally meant to be in Super Mario Bros. 3 (as seen on the picture on the right), released 21 years prior, where these enemies would have come in groups and would have swam faster.