Werething is a catch-all term used by the locals of Upper Posada to describe a variety of (mostly imagined) creatures which plague the area.

Aaaa. The alderman quite visibly started to think. Work? Maybe those... Well... werethings? You're asking me are there any werethings hereabouts?

— page 164, The Last Wish (UK edition)

"Known" werethings include:

... and bats