Luigi's egg form, seen in only one episode of Super Mario World, is the evolved form of Stone Luigi. He was attacked by a Koopa Wizard! (That's WIZARDHEIMER!) Wizenwhatever. Egg Luigi is great to use as a football, but don't drop him, or else "Luigi get scrambled," as Yoshi claims.
Then they proceeded to play football with Luigi, seemingly caring little for the fact that he could be lost permanently. After a while, Luigi hatched as a fully grown, yet retarded man.