The Argonaut, or Paper Nautilus, is a small octopus found in the open-ocean. Like many octopi, the male Argonaut’s third left arm develops into a “hectocotylus,” (octopus penis) which the male is able to detach for copulation. What makes the Argonaut’s hectocotylus unusual is that once detached it can swim on its own to the female, where it then attaches within her “pallial cavity” (octopus vagina).

Argonaut appears at the 32 second mark and proceeds to demonstrate his prowess

I can't help but muse that this phenomenon lends credence to the notion that penises have “minds of their own”.