6x23 - The Grim Adventures of Billy and Mandy

As Billy notices Mandy and Grim's usual negative attitude, he wishes they were in a magical land of happiness, so Grim transports them to an enchanted forest. Billy enjoys the relaxed surroundings until he meets a cantankerous and sour unicorn named Mary-Frances. Billy tries to change Mary-Frances attitude, but she is reluctant due to an earlier trauma of not being able to swim and follow the other unicorns into the sea. After Billy teaches Mary-Frances to swim, she feels happy and finally can join the other unicorns, but they turn out to be bullies, the real reason she never joined them in the first place. Billy stays with the unicorns and Mary-Frances goes back with Mandy and Grim to watch TV.