A Constrictor Snake with the ability to camouflage itself as various kinds of rope suited to their environment. In the wild, Rope Snakes are better known as Vine Snakes because they take on the appearance of vines hanging from the trees. It is not uncommon for them to captured and deployed in dungeons as traps. They disguise themselves a suitable rope and are trained to wait until an inopportune moment to make themselves known.
I can’t remember why I added this to my list, but I loved it in the Labyrinth of Eternal Horror and Also Knives.