I am in the works of designing a game and I want to add some easter eggs. For example, one of the eggs is that when the player finds this hidden area they get to meet the "grand cosmic owl". This experience is very brief and made for kicks. Thing is, "the grand cosmic owl" is a cartoon show reference from Adventure Time. Do I need licensing to use their character?
I know games such as zelda easter egg mario and mario games may easter egg Samus aran. I assume its because these characters are all Nintendo.
Asides from a cartoon show character, what about book characters, movies characters, etc...? Also, I am looking to monetize of my game.