It is well known among today's Gaijin gods, and some goddesses, that Tentacle Sex was invented by manga artist Toshio Maeda.
However, today I just discovered an old 80's era Japanese TV commercial for Shin Piro Ace athete's foot ointment that, possibly, could have been an early influence on Maeda's work:
You can see two short interviews with the master himself, Toshio Maeda: Hentai Pioneer and Hentai Pioneer II. And be sure to see more at TokyoDV Video Log