Analogous to Rules 34 through 37, Rule 38 states that if something exists, there is a plushie of it, or there will exist a plushie of it. This is not a constant, but an inevitability; i.e. not all things have plushies, but come the end of time, they will.
"Half-Life 2: Episode Two had those really scary Hunter enemies; now they have them as plushies."
"Christ, Rule 38."
"WTF is rule 38?"
"It's like all the other Rule 3x's, but about plushies."