From the Super Mario Wiki, the Mario encyclopedia
Gargantuan Gills are gigantic aquatic predators found in a few vehicle-riding stages of Donkey Kong Country: Tropical Freeze. One of their key traits is the row of razor-resembling appendages going along their spine. They also have black rims around their eyes, similarly to Punch Piranhas, another predatory species from the same game. Gargantuan Gills behave differently depending on their color.
Light-blue Gargantuan Gills are only found in Twilight Terror, where they often emerge at the surface of the water and move in tandem with the Kongs, who fly a Rocket Barrel above the river. They travel in-between rock outcrops and often collide with underwater items, which are then casted towards the Kongs. They tend to knock over structures and cause them to potentially fall on the playable characters, damaging them. They can sometimes cause direct harm to the Kongs by swimming in front of them with their sharp appendages. Near the end of the level, a light-blue Gargantuan Gills is piqued by the Kongs and chases them down the river, only to fall down a cascade into a deep pond below. The Kongs can find it again if they dive to the bottom of the pond, where they encounter the fish immobilized and unable to cause harm.
Purple Gargantuan Gills are more aggressive and jump high above the water, trying to snatch the Kongs before diving back into the river. Aside from Twilight Terror, two of the purple variety make a minor appearance in High Tide Ride, where they unexpectedly jump out of the water to bite significant patches of railtrack and force the Kongs to jump over the newly-formed gaps as they ride in their Mine Cart. A Blue Hootz is eaten by one of these creatures in the process.