Cheep-Cheeps are enemies in the Mario video game series, first appearing in Super Mario Bros. for the NES. They are the most notable aquatic creature in the series, and have appeared in most of the Mario video games, where they heavily resemble fish. They usually appear in the 2D series of games, but only have minor appearances in the 3D series. In Super Mario Sunshine, you would be able to turn them into platforms by making Yoshi spray juice at them once they jumped out of the water.
They apparently have their own language according to Paper Mario, though they've managed to learn English to some extent, as some have been able to do so, albeit not perfectly seeing that they oftentimes mess up the words.
It is evident that Cheep Cheeps are food to the people of Mushroom World, just as fish are food to us in our world. In the track Shroom Ridge, business trucks can be seen driving on the road. Some of these business trucks have a picture of a Cheep-Cheep on the side, with the word "FRESH" over the picture. This would of course mean "FRESH Cheep Cheep", indicating that they are a food that is eaten.