Tentacle Monster

The tentacle monster from anime hentai is probably the most well-known use of tentacles. The form of the tentacle monster can be just about anything, from a demon monster, or a plant of some sort, or a sort of sea monster (such as the Giant Squid in Harry Potter fandom, which might be considered a tentacle monster when put to use for kink).
The purpose of the tentacle monster is much more relevant than its specific form: it has tentacles, lots of tentacles, sometimes blatantly resembling phalli, and it uses them to probe every orifice of its helpless victim. In hentai as well as fanworks, penetration is usually involuntary, and tentacle rape is a common term which goes with the tentacle monster. The tentacles also restrain the body and put it on display, and voyeurism by another character (by a hidden character, by another character also being subjected to the tentacles, or by a character controlling the tentacles) is a common trope for this genre.
The tentacle monster is a source of great amusement for anime/manga fandom, and is often brought up in a joking manner and is subject to much parody. Fanfic featuring tentacle monsters tends to be tongue-in-cheek. Doujinshi might have more serious and dramatic depictions, as far as porn goes.
The tentacle monster first gained major popularity in the 1990s with the anime version of Urotsukidouji (Legend of the Overfiend), which was both an adaptation of the manga series of the same name (which did not feature tentacle porn) and inspired by Youjuu Kyoushitsu (Demon Beast Invasion), another manga series by the same author (which did). Among other things, tentacle porn was appealing for anime studios because it allowed them to get around Japanese censorship laws prohibiting the full exposure of the penis. However, Japanese erotica depicting sex with monsters and tentacled creatures long predates 1990s censorship laws.The Dream of the Fisherman's Wife, a woodblock print from circa 1820, depicts a shell diver receiving oral sex from an octopus, for example. This print would have been neither the first nor the only of its kind.
Inkstone briefly discusses Urotsukidoji in her picspam of Claymore's tentacle monsters:
:Now, if you have any knowledge of tentacle hentai, you may have heard of a title called Urotsukidoji (aka Legend of the Overfiend). It gained infamy during the 80s and helped to fuel the "OMG ALL ANIME IS PORN!! SAVE THE CHILDREN FROM THESE SCANDALOUS JAPANESE CARTOONS!!" panic of the time. Of course, it didn't help matters that a good chunk of the anime and manga coming over to the U.S. at the time was pornographic in nature but obviously, not all anime is porn. Some of it is but not all of it.
:Anyway, Urotsukidoji contains what has now become an iconic scene in which a hospitalized young man awakens, becomes a monster, and assaults the nurse on duty. See the callback right there? In tentacle hentai, where body horror often takes center stage, a person awakens -- either literally or figuratively -- and becomes a monster. Just like the Claymores.
:Because, you see, what made that scene in Urotsukidoji go down in infamy was that the nurse gets torn apart at the end. All similar scenes in tentacle hentai that have come since all pretty much go back to that scene. And these panels from Claymore are referring to that.Devour: a tribute to the female tentacle monsters of Claymore (Accessed May 11, 2011)
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