Kappa seem to show up quite a bit in popular culture, or at least the small sliver of popular culture with which I am familiar. While I can't recall any of them actually appearing in the Harry Potter books, they did make it to the tie-in Fantastic Beasts and Where to Find Them. Final Fantasy VI has a spell that will turn enemies into kappa, and apparently kappa without water on top of their heads, as they're quite weak. In the Super Nintendo version (haven't played any of the others yet), the creatures are usually just called "imps," but the one who teaches about Gau's monster-imitating techniques is identified as a kappa.
I've also seen some suggestions that Mario's enemies, the Koopas, have a name that's a play on "kappa." After all, they're also malevolent turtle-like animals. Actually, I understand that the more likely reference is to "kuppa," a kind of Korean soup, but maybe it's a play on both of them at once. Speaking of which, the Super Mario Bros. 3 referred to Kuribo's Shoe ("Kuribo" simply being the original Japanese name for a Goomba) as a Karubi Shoe. In the episode "Super Koopa," Bowser uses the shoe to try to crush the Mario Brothers, causing Mario to exclaim, "I don't believe it! Karubi Koopa!" And just today, I learned that karubi-kuppa is beef soup with rice, so what I thought was just sloppy writing might have actually been sloppy writing for the sake of a pun. Anyway, there is a clear reference to kappa in the Mario series, in Kappa Mountain on Yoshi's Island, given that name because the lake on the lower summit is similar to the water-filled hollow on a kappa's head.
By the way, thanks to vilajunkie for the link to the Obakemono Project, which provided a good source of information on kappa. Other sources consulted were Wikipedia and the Super Mario Wiki.