I don t remember what this movie is called?

Okay, so when I was really little I watched this movie, and I m really annoyed on how I can t remember the name, It was a weird animated movie about monsters, and there was two main monsters, there was a blue one and a red one I think, but basically they find this child, and like they take care of the child, but the blue monster really didn t like the whole thing, and at the end the blue monster decides to run away, and then red monster finds where the child lived and returned him/her