Tentacle Monster

The Tentacle Monster is a specified type of creature that is defined by their sexual appetite and their possession of tentacle appendages. While exceptions do exists, these creatures are largely depicted as sexually predatory and completely alien in comparison to the human form.

History
The tentacle monster's prominence in Japanese erotic literature largely arose due to influence from the aesthetics of censorship in western civilisation.

As part of the Meiji Restoration (1868-1912), which saw Japan adopt many aspects of western civilisation in order to "become more civilised", Japan adopted new laws influence by Victorian-era Europe to forbid public nudity (where previously Japanese men and women were free to walk around topless or in loin-cloths should they wish) and multi-sex public baths. During this era, creation of traditional Shunga art (erotic artwork that was commonly accepted as non-taboo in Japanese life) was also restricted.

Restrictions increased further during the years surrounding World War II, forbidding all erotic art and literature, until post-WWII when the occupying Allied Forces imposed a number of reforms on the Japanese government; which included a lessening on restrictions of erotic expression. Due to this, while “obscenity” (which included depictions of exposed genitalia) was still illegal to portray without censorship (which led to the creation of the "Black Bar" and "Mosaic Blur" to censor the offending material), the diversity of sexual acts that were now permissible to use led to a rise in erotic fiction. By 1990, in their pursuit to create new material, erotic artists had devoted their efforts in creating new ways to either circumvent the "obscenity law" or to otherwise eroticize otherwise banal subjects, however, it was artist Toshio Maeda who is credited with coming upon the notion that, while the law forbid depicting traditional human genital-to-genital intercourse, it did not forbid depicting pseudo-genital-to-genital intercourse. Following this revelation, Maeda creates a new erotic manga that depicts women in sexual situations with non-human creatures who use tentacle-appendages to penetrate and otherwise assail the female characters. As the monsters did not have a defined gender and did not have/use human genitals (as they are strictly defined by the law), the sexual acts could be shown uncensored. Furthermore, the diverse sexual utility that these fictional creatures brought to the conceptual table meant that numerous fetishes could be believably implemented at the same time.

Trivia

 * One of the earliest examples of Tentacle Monster erotica is a shunga illustration from the 1814 novel Kinoe no komatsu; popularly known as  where a Japanese pearl diver is depicted having sex with two Octopuses.