I remember a rumor that Donkey Kong appeared as a hidden boss in either Super Mario RPG, the first Paper Mario, or maybe M&L: Superstar Saga. Also, hoaxes about Mr. Resetti's House.
After online guides became widespread, I did end up catching a Mew in Pokemon Blue, following a guide from GameFAQs. It's not under the truck, though.
If I play a random game I find on Itch, it's likely harder to find videos/guides where someone else already found all the secrets. The tradeoff being that with fewer players, there are fewer entertaining rumors about obscure games.
BTW, did you know that in Tomato Clinic, it's totally possible to unlock the ability to play as a werewolf? My Aunt who works at NPCKC said so. Vampires don't accept blood donations from werewolves due to centuries of conflict between their species.