I mean, I guess "PwH" is supposed to stand for "Prismriver with Horikawa" ??? Like, can't name actual names kind of situation, so use an abbreviation people will pick up anyway?

Of course, it's a Pixiv article...

Oh? It seems ZUN himself confirmed they switched from "Horismriver" to "Prismriver with H" because those three didn't like the name that Raiko just chose by herself without their input.