Gloop is a friendly blue fish from Clanker's Cavern in Banjp-Kazooie. He supplies Banjo with air in the form of bubbles while the bear tries to raise Clanker by swimming through a key deep under water.
Many players believe that the fish found in Hailfire Peaks, simply called "Din-Din" by Boggy, could in fact be Gloop because of its striking resemblance to the bubble-blowing fish.