Deep Cheep

Deep Cheeps (originally parsed Deep-Cheeps ) are a special variety of Cheep Cheeps that are first found in New Super Mario Bros. They appear to be regular Cheep Cheeps, except for the fact that these creatures are green and will actively chase Mario or Luigi in underwater levels depending on the direction they swim in. Deep Cheeps will only give up pursuing Mario or Luigi provided the latter ones swim behind and past the fish foes.

Deep Cheeps can be confused with the green Cheep Cheeps from Super Mario Bros. 3 and Mr. Kipper from Super Mario RPG: Legend of the Seven Stars. Despite this, Deep Cheeps have a unique characteristic - that is, swimming in the direction of the player character with an angered expression.

New Super Mario Bros.
In New Super Mario Bros., Deep Cheeps and their larger variants, the Big Deep Cheeps, only show up in World 6-5. They chase Mario or Luigi until the heroes swim away.

New Super Mario Bros. Wii
In New Super Mario Bros. Wii, Deep Cheeps are slightly more common. One appears in World 1-4 along with a Big Deep Cheep. If Mario and co. keep staying in the area, another pair of green fish enemies appears. Deep Cheeps can also be encountered in the second part of World 8-4.

In this game, their lips have become pink, like those of a Cheep Cheeps, rather than keeping the red-orange colored lips they previously had. Their dorsal fins and tails have also become yellow, as opposed to white. There is a more relentless enemy similar to Deep Cheep known as Spiny Cheep Cheep.

New Super Mario Bros. 2
Deep Cheeps reappear in New Super Mario Bros. 2, where nothing has changed about their behavior and appearance since New Super Mario Bros. Wii. Deep Cheeps first appear in.

New Super Luigi U
Though Deep Cheeps did not appear in New Super Mario Bros. U (despite them receiving official artwork for that game), they still appear in New Super Luigi U, behaving the same as in previous games. Their only appearance is in Cheep Chomp Chase.

Mario Party 9
In Mario Party 9, a Deep Cheep can be seen swimming around Blooper Beach. It jumps out of the water at various intervals and may even jump over the player's vehicle. It has no effect on the gameplay.

Super Mario Maker / Super Mario Maker for Nintendo 3DS
Deep Cheep reappears in Super Mario Maker and Super Mario Maker for Nintendo 3DS. They aesthetically replace green Cheep Cheeps and Blurps in the New Super Mario Bros. U style, except that they no longer chase the player due to the green Cheep Cheeps and the Blurps from other styles not being able to chase Mario. Furthermore, the voice heard when placing Deep Cheeps onto the course just calls them Cheep Cheeps.

Trivia

 * Although Deep Cheeps did not appear until New Super Mario Bros., an unused green Cheep Cheep sprite does exist in Super Mario Bros.