Scuttlebrigs were creatures introduced in The Descenders. They could camouflage themselves to become invisible when seen from above, making them extremely valuable for people trying to sneak past Quove Lentis' barricade and into New Sanctaphrax.
Behind the scenes[]
In Chris Riddell's book Travels with My Sketchbook (Macmillan, 2017) appeared various sketches from the black books. One of them showed a scuttlebrig. The original name for the scuttlebrigs seems to have been Leatherbacked mudcrawler.