usually) body. Occasionally it may be mutual pleasure, as in The Dream of the Fisherman's Wife, often considered the Ur-Example. The appeal probably comes from how tentacles can constrict movement, can enter any orifice, and are much more like a tongue than a bony limb (though in this case they may allude to penises instead).
This type of erotica is associated with Japanese manga and OVAs, particularly the works of Toshio Maeda. However, it has always been a very small niche even in hentai, but a few infamous releases in The '80s resulted in the perception that All Anime Is Naughty Tentacles, which is when people expect anime to be this trope if not hentai. Similar motifs have occasionally appeared in erotic art from around the world.