You can keep a dog as a pet or else a cat.But here in this movie a lonely boy Keeps the legendary monstrous "Water horse" as his pet.He happened to see an egg from ocean that no one cared in the shore.He took it with him without knowing what is inside.
As logo says how he kept such a big thing as a pet in water.

But you see this is a fiction and anything happen.
Watch the movie for the sequence of events faced by the boy as a result of his "pet".

Related Movie : Dwar