Oh I have, many times. In middle school or high school, we didn't always have copies of the rulebooks to refer to, so we had to make a lot of stuff up on the fly. And even more recently in the mid-2010s, I would babysit my niblings with an occasional game of D&D.
Nibling: (looking through the Monster Manual, stops at the painting of Rivergleam the Pixie) Oooo, she's pretty! Can I be a fairy princess?
Me: You can be whatever you want! (helps them add the pixie stats to their character sheet)
Nephew: Anything? Can I be a dragon? I wanna be a magical dragon!
Me: That sounds really awesome, let's do it! (helps him roll up a dragonborn wizard)
Rules have their place, but they aren't the end-all, be-all necessity that some folks paint them up to be.
EDIT: Woah. I just realized that "dragonborn wizard" graduated high school last Saturday, and his "pixie" sibling is only two years behind him. Time flies when you're rollin' dice.