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. 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.

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.