The mantacorn is a very dangerous and fierce creature that lives in Merroway Cove. Every year, he sheds his horn and keeps his old horns as a private treasure. According to Sven and Slim, his favorite food is sea horses. According to Oona, he's so protective of his old horns that anyone who enters his cave never comes back out. He first appears after Fluke's recklessness wakes him up. Realizing that they came into his cave to steal one of his old horns, he chases Fluke, Sofia, Oona, Sven, and Slim all around his cave, to Sven's dismay, to attack them. After grabbing one of his old horns, Fluke tricks him into chasing the quintet through the sea weed patch outside his cave and gets him stuck.
Stuck in the seaweed, the mantacorn demands that they tell him what he did to them. After Sven tells him he tried to kill them, he explains that the reason he was so mad was because he hadn't done anything to them but they still stole one of his precious old horns. Sofia explains that they only did that because they needed one of his horns so the Mer-Tones could play their music. The mantacorn tells them all they had to do was say so and ask him for a horn because he loves music. After Sofia, Oona, and Fluke get him free from the seaweed, he tells Sven and Slim that the belief that he eats sea horses is just an old merwives' tale and he actually eats puffer fish, to the sea horse's relief and the puffer fish's horror. He is later seeing performing with the Mer-Tones for Sofia's family.