Post-Credits Scenes Before the MCU

[Rincewind]

Airplane was the earliest I can think of.

Constantine had a post credit scene.

[Wildling]

James Gunn's horror movie Slither had one 2 years before the first Iron Man movie came out.

[lalalei2001]

Super Mario Bros. had Iggy and Spike pitch a video game to Japanese game designers about 'their' adventures, the Super Koopa Cousins.

EDIT: Here's a list! It looks like the first one ever was from The Silencers. https://en.wikipedia.org/wiki/List_o...credits_scenes