If it exist's, there is a pony variation of it.
Person A: "What if there was porn, but with ponies?"
Person B: "Rule 34.5"
If it exists, there is a subreddit of it.
Anon: Nice picture of a penny. Don't you think that belongs to /r/numismatics?
OP: I didn't even realize that exists!
Anon: Dude, Rule 34.5