From The Heretic Knowledge Vault
A kind of familiar, represented in the story by Bani Igaaru's familiar Rape-kun, which see. Like any familiar, a micro-tentacle monster has the job of providing companionship and comic relief to a mage. Rape-kun, the only one we've seen so far, doesn't have the same communications skills as other familiars like Ellis and Locke, but that's probably just as well, and Bani seems happy with him despite his occasional misbehavior.
Micro-tentacle monsters are generally octopus-like in shape, with a small, spherical body and a number of legs/tentacles. Art in the main story leaves it uncertain just how many tentacles they have and whether the number is fixed or variable, but in Rape-kun's appearances in the "Fun with Familiars" filler art, he always has nine tentacles.
The fact that this beast is called a "micro" tentacle monster may imply that there are macro tentacle monsters out there somewhere. If so, you probably don't want to run into one.