My thoughts on this are that there will be a 'parts' warehouse...and the game just picks a series of numbers...equatable to a position on a body...or plant...and it attaches said items in interesting ways.
How big the warehouse will be is going to be a challenge for any game....but the idea is fairly straightforward.
(Now, I have problems with Spore....but the way they handled critter creation was interesting...and would be kind of fun if NMS or E: D added player capability to make parts for the game.)
Parts warehouse is a possibility, definitely. In the case of NMS there's actually a Game Informer interview back in 2014 that dug into the nitty gritty behind their work on the game, some of the reasons they decided on things like having planets closer together, etc. When it came to creatures, plants, etc, (to paraphrase from the video,) Sean essentially said;
"So when you make a character in an MMO or that kind of game for the first time, you have all these sliders you can use to make that 'your' character. We have something sort of like that in game for everything. Every creature, tree, rock, grass... so the artist will build something like this-" *gesturing to a random creature on the screen* "-...actually, it won't be this one, you know, they'll build something that's just a prototype, then they model it out in something called (ZedBrush? ZenBrush?) and you're basically just making things out of clay. So they don't so much model in polys or things like that, and they don't do texturing and things like that so much. And then they bring it into game, and they can click a button and it will show them hundreds of variants. And then they can click it again and it will show them hundreds more. And this will be just one creature type, and there'll be hundreds of creature types in game, and each one will have a practically infinite number of variations."
I'll include the link below, but my interpretation is that the artist basically comes up with the rough shape of each creature type that are very roughly modeled, and probably already given a set of animations. When they run it through the generator, it will adjust such things as size, proportions, textures, colors, possibly additional (maybe non-animate) appendages like horns, even finer details like the shape of the head or whether the limbs end in feet, hooves or hands; and I believe that it will even make small adjustments to animation depending on other details in the variant. So very large variants will generally be programmed to move with a slower, lumbering gait appropriate to their size, whereas a much smaller variant could scuttle more quickly. What's interesting to me is that, when he was clicking through some of the variants for one specific four-legged shape, the variants seemed to range from rodent-like creatures to lizards, so there seems to be a lot of 'wiggle room' in a given creature type.
I think the interview was linked a couple of years ago, buuuut I'll link it again. =D Roughly 21 minutes in is where he starts talking about procedural creatures.
http://www.gameinformer.com/b/featu...te-behind-the-scenes-tour-of-no-mans-sky.aspx