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.