The skriton. Is is an arthropod? I don't believe so.

Yes, it is shaped rather similarly to a scorpion. I'm not disputing that.

However, this thing has TEETH. And a full on mouth, and it roars, and you can see its slimy pink throat when it does so. Athropods do not work that way!

Ergo, I submit that the skriton is either a reptile or a mammal. It's hard to determine which. Its carapace may be reptile scaley skin, or it may be akin to a pangolin's armor-like scales. Roaring is generally more of a mammal thing but reptiles can vocalize so it's not a dealbreaker. As for the scoprion-like shape and attributes: I say it's merely convergent evolution!

If you want to get fringe with it, it may even be some kind of land fish. But an arthropod? Nay, I say! Impossiblay!