Big Deep Cheep
From the Super Mario Wiki
Big Deep Cheeps, originally Mega Deep-Cheeps, are large Deep Cheeps that make their appearance in all five New Super Mario Bros. games. In every one of these games, they are merely bigger than their parent species, but act the same nevertheless: a Big Deep Cheep swims straightly forward, until Mario and his friends approach its vertical axis. If so happens, the fish enemy will try to rapidly swim towards the heroes in an attempt to hurt them. However, if the player-controlled characters manage to swim over the foe, it will stop its pursue and continue its aimless underwater rambling.
Compared to their appearance from New Super Mario Bros., Big Deep Cheeps in New Super Mario Bros. Wii had their size reduced so they appear to be just slightly bigger than common Deep Cheeps. However, in New Super Mario Bros. 2 they approximatively regain their original size.
In Super Mario Maker, the player can enlarge a regular Deep Cheep by placing a Super Mushroom onto it in the Edit area. It thus becomes a Big Deep Cheep with the size of those from New Super Mario Bros. Wii. In this game, Big Deep Cheeps have lost their Mario-chasing instinct, and instead follow the same strict movement pattern of green Cheep Cheeps from Super Mario Bros. and Blurps from Super Mario World, as they swim straight without turning back. Big Cheep Cheeps, on the other hand, turn around after they have traversed a certain distance.
In all of the mentioned games, Big Deep Cheeps can be defeated with the power confered by a Fire Flower or a Super Star. Exclusively in New Super Mario Bros. Wii and New Super Luigi U, players can transform these fish foes into frozen blocks with the ice balls given by the Ice Flower or the Penguin Suit. Likewise, only in New Super Mario Bros. 2, players can acquire Raccoon forms and down Big Deep Cheeps with tail swings.