The Roper is an iconic Dungeons & Dragons monster; an ancient entry to the game, it is the most long-lasting of the plethora of "killer shit in disguise" monsters that proliferated during the days of Advanced Dungeons & Dragons.
In a nutshell, a roper is a predatory aberration that resembles an outcropping of rock with one eye, a giant mouth full of teeth, and long, sticky tentacles it uses to grab victims and drag them into range of its mouth.
In some editions, the juvenile form of the roper is the piercer, a snail like creature that hangs from the ceiling of caves pretending to be a stalactite, and drops on people that walk under it to try to impale them with its body.
Lords of Madness reveals that ropers can undergo the same process used to turn humans into illithids, which results in a creature called an Urophion.
The abundance of tentacles has made the roper the go-to "tentacle monster" for vaguely D&D-themed /d/ manga and anime.