Eep Cheep

An Eep Cheep is a variant of Cheep Cheeps first found in New Super Mario Bros. Wii. They are similar in appearance to Cheep Cheeps, although they are yellow in color, 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 fright in reference to their "cowardly" demeanor.

New Super Mario Bros. Wii
Eep Cheeps make their first appearance in New Super Mario Bros. Wii. They will swim away from the player whenever they get close, which is the the opposite of Deep Cheeps and their behavior.

New Super Mario Bros. U
Eep Cheeps return in New Super Mario Bros. U. They retain the same behavior and appearance.

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

Appearances in non-Mario media
In the Super Mario Mash-Up Pack in Minecraft, Clownfish are replaced with Eep Cheeps.

The twelfth Fishing Tourney in Animal Crossing: Pocket Camp that began on March 14, 2019 featured Eep Cheeps alongside regular Cheep Cheeps, Cheep Chomps, and Bloopers, owing to it being run on Mario Day. An Eep Cheep balloon attached to a Brick Block could also be obtained as special furniture after reaching the 800, 950, 1,100, or 1,300 cm milestone.

Trivia

 * Though first seen in New Super Mario Bros. Wii, the concept of yellow-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 of three and swam faster than green ones. While they are technically more of a tan color in the original due to sharing a palette with Goombas, Super Mario All-Stars and Super Mario Advance 4: Super Mario Bros. 3 color it yellow despite it still being unused.
 * Eep Cheeps are unused, but fully working, in New Super Mario Bros. 2. They function just as they did in the games they appeared in.