There are tentacles, whips, and penile limbs throughout Giger's body of work. From the ALIEN concept art, none I can really think of, though Giger painted 30+ pieces for it and most are in the Giger's Alien book and not really around the web. The tail was only added to the original Alien because Ridley Scott requested it, so Giger drew it (same for the mouthed tongue - Giger's Necronom inspiration for the Alien boasted two rows of teeth overlapping the other, almost like a shark. Ridley wanted it on the end of a tongue, which Giger went ahead and drew). Another Medusa-esque creature with tails and whips and penis-tentacle hair etc:
Spoiler
Note the large tentacle appendage running down the middle of the creature, in addition to hoses, pipes, and worm-like appendages.
You;d have to scour Giger's published books for more. There are a lot! Giger always held a macabre fascination with worms, he says in the Necronomicon, and he drew on the imagery of them since his beginnings as an artist. I think the tentacled monster in Prometheus is more drawn from Cthulu and O'Bannon's fascination with the Lovecraft mythos.