Roper
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.
The abundance of tentacles has made the roper the go-to "tentacle monster" for vaguely D&D-themed /d/ manga and anime.