The biomechanoid that O'Bannon wanted

Leading from:
The Alien creature

detail from Necronom V. Was it this one?

a) Mystery of O'Bannon's original chosen biomechanoid
Instead of Ron Cobb's earlier design for an alien creature that resembled a sort of an oversized lobster, Dan O'Bannon had a vision Giger's alien creature in mind for the monster right from the beginning as he sat on Ron Shusett's sofa. He would describe a Giger painting but exactly which one it was is not easy to say. Describing a Giger painting at first glance can be very hard indeed and through words one might only convey vague impressions. Which one he was talking about? He describes it has having a head distortion towards the front.

Homage to Becket I, work 93
Is this the one?

b) Not going to O'Bannon's plan
When Ridley took a look at Giger's Necronomicon and found a different source of inspiration, namely the huge phallic demon in Giger's painting Necronom IV and it had the distortion towards the back, as we see in the final creature. Dan O'Bannon's intended one had the distortion of the human head body coming out to the front as if to say he would have have preferred the back pipes coming out of the front of the chest. However he felt that it was best to leave it to the director to go his own way with the movie.

c) Issue resolves itself
As Giger got to work on adapting, designing and shaping the creature, without Dan having to direct him down any particular direction, almost by coincidence it got to a point where Dan was very surprised by the results and he thought to himself "you know, it's amazing. Damn it, it's even similar to the one I've been thinking of. " Still if Dan had been directing he would have wished to have Giger do something to break up what seemed too much like a human silhouette a little more, give the thing six legs instead of two or something like that.

detail from The Spell II.
 Is this the one?

Source quotes

1. Dan O'Bannon: Well another source was that I met Giger when we were working on Dune, and I'd looked at his picture books and when I got back to America I was still haunted by his work. It was on my mind and when we sat down to do Alien I ended up visualizing the thing as I was writing it, I found myself visualizing it as a Giger painting. And I wrote this script. (Fantastic Films us #10/ GB no1, p13)
2. Dan O'Bannon: I had this vision right on this very sofa, of Giger monster around with a science fiction horror movie could be based and it ended up happening. In fact the design that they ended up getting, almost by coincidence, I had settled on in my own mind. One of Giger's designs that I liked and I wanted to see as the monster.  Later on Ridley went through Giger's work and he found quite a different source of inspiration and he had Giger design from that. But the funny thing was, when it got done - when Giger ended up adapting it and designing it and shaping it up - it ended up being similar to the thing that I'd had in mind that I never mentioned. I was just so happy to get Giger that anything he did was fine by me."(Fantastic Films us #10/ GB no1, p13)
3. Dan O'Bannon: When I started thinking back. I said "you know, it's amazing. Damn it, it's even similar to the one I've been thinking of."  There's a head distortion on the creature, the one I wanted distorted the head toward the front. The one that Ridley picked distorted the head out towards the back, they're in the same family. (Fantastic Films us #10/ GB no1, p13)
4. Interviewer: What did you see in Geiger’s ‘Four and Five’ paintings?
   
   Dan O'Bannon: Well, Ridley went through the book, I gave him a copy of the book. Finally the way I was able to get Geiger on the picture was by showing the stuff to Ridley when he was hired, cos Ridley went for him. Well, I’m not sure I’m answering what your real question is, but I will tell you that simply I had one of his paintings that I thought Giger could base the alien on and Ridley had a different painting in the same book that he thought he could base it on, and the most important difference between those two was that in the picture that I chose the distortion of the human body stretched out to the front and in the one that Ridley picked, it stretched out to the back. That’s about the biggest difference I can see between Ridley and my preferences to which one of Giger’s paintings was the best inspiration as it were.
   Also, if I’d been directing the thing, as an old sci-fi buff, I probably would have asked Geiger somewhat of a less of a familiar human type silhouette. I wanted it to be alien. Even though it was a man in a suit, I wasn’t particularly happy about the fact and if I could have given it six legs or something . . . But I wasn’t that, once somebody else is directing a movie you don’t bother with that kind of detail, I’m a firm believer in leave the director alone. He’s got enough trouble without the writer running forward and telling him but you changed the character’s social security number! See, even if the director’s wrong, even if he needs to be straightened out by the superior knowledge and creativity of yourself, the writer, even if that’s the case, he’s so swamped with having to do his job, he can’t listen to you. All you do is make it tougher on him without getting any benefit out of it.
   So I pretty much backed off. Especially cos the different people working on that picture, I mean some of them the quality of the work was just so dreadful I was not gonna spend my days beating up on Ridley cos he wanted the pipes to come out of the back instead of the chest. 
   (report from unused part of interview for Alien Evolution documentary)