(with some of these being told to me by my aforementioned friend — they aren't in a particular order, though)
Rhydon was the first Pokémon ever designed
Mewtwo, statistically, has the highest base stat total of any Gen 1 Pokémon
In Gen 1, a glitch made it so that Psychic types were immune to ghost type moves instead of being hit super effectively by them
Initially, Arcanine was going to be a legendary Pokémon, but that didn't end up happening
The games were based off of creator Satoshi Tajiri's hobby of bug catching as a child
Clefairy was actually originally planned to be the game's mascot instead of Pikachu. I'm pretty sure that should be illegal
The player's rival in Gen 1, Blue, is actually called "Green" in Japan, and similarly, instead of the games being "Red and Blue," they were "Red and Green"
There was an extremely popular fake story that the original "Lavender Town" music led children in Japan to kill each other
Jigglypuff has drawn mustaches on people because they fell asleep during their performances. You don't want to do that
There was a scrapped Professor Oak final battle that originally took place, but that was cut from the final game