http://tvtropes.org/pmwiki/pmwiki.php/Main/NaughtyTentacles

Some Horny Devils and Eldritch Abominations have nothing on their minds but sex. They want it, they want it in quantity, they want it now, and they want it from nubile young women (especially beautiful and cute Japanese schoolgirls). To this end, they almost always manifest in forms that have many, many tentacles - tentacles that are used to violate women.

See also All Anime Is Naughty Tentacles, which is when people expect anime to be this trope if not Hentai.