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 word "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 appear in groups, called Eep-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.

Trivia

 * While New Super Mario Bros. U officially names the enemies Eep-Cheep, they were referred to as Golden Cheep-Cheeps in New Super Mario Bros. Wii.
 * 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 this enemies would have come in groups and would have swum faster.