In his book Anvil of Stars, the human characters meet up with a species they call the Brothers. Each one is sort of like a large centipede (terribly creepy at first!), but their bodies are composed of a number of smaller sub-organisms, sentient but non-sapient on their own, that combine to form the "individual," so to speak. They struggle a bit with concepts like singular pronouns, and always refer to themselves as "I/we". A big part of their communication is olfactory, and the odors they give off don't always have the kinds of associations we humans might expect, which is fun. They're ultimately very relatable, though, once you get over the "eek!" factor. I really love these guys, and I'm not doing them justice here.

As a side-note, the humans in that book are a little alien themselves, since they've all grown up on an alien spacecraft that has honed them for a single purpose, which is to hunt down and destroy the race of Killers that wiped out the Earth and a bunch of other planets besides. (The Brothers are on the same mission.) It's a little bit Ender's Game, but Bear acknowledges that space is really, really big, and they're essentially tracking the Killers back through eons, and they might not ever find where they came from; or if they do, they might be long dead already.

Anyway, the culture the kids form among themselves is unique and pretty interesting, drawn on half-remembered childhood stories and images of Earth. Their leader is the Pan, and they call themselves Lost Boys and Wendys. They organize themselves into families such as Trees and Birds, with each kid having their own byname that fits their family along with their given name. Martin Spruce is our protagonist; Rose Sequoia is another I recall. Expression of sexuality (they're physically adults at the time of the story) is much less rigid.

TL;DR, Anvil of Stars is cool, go read it.

~Neshomeh

I speculate it's easier to put together something properly weird and unusual when you don't need to actually show and play it to people. You can just try and use words to push folks imaginations into roughly similar spaces.

One example that might sort of work on film are the Tines from Vernor Vinge's A Fire Upon the Deep, who're dog-like beings where the sapient people are made of four or five individual beings who synchronize together with ultrasonic waves and who aren't individually sapient. This leads to interesting (and plot-relevant) abilities like the ability for a Tine to swap out parts of themself.

And then you have a book like Embassytown, which features the Hosts, who're aliens that, for reasons that were either not specified or that I can't remember, have a language where you're always saying two words at once. Some aspect of how this setup interacts with their brains stops them from lying or using things like metaphors. Interspecies relations rely on genetically engineered twins who can more or less speak this language, since regular human languages don't cross the species barrier. (And then things get weird and plot happens.)

Anyway, these're some examples I could think of right now. Might come back with more later.

More specifically, a book series called 'The Lost Fleet: Beyond the Frontier' (which is itself a sequel to a series called 'The Lost Fleet') by Jack Campbell. In it, first of all, all space battles are 4D, so not only do you have to figure out where the fleets are in space relative to each other, and the system they're fighting in, but also *when* which makes those battles both truly complicated and truly awesome and even now when I read through some of the more complex battles for the umpteenth time I have to put the book down and try to imagine the battle in my mind before continuing on. Could you pull that kind of experience through to the big screen? I don't know. But in book form it is pure awesome for any sci-fi fan.

Secondly the aliens in 'The Lost Fleet: Beyond the Frontier', if after reading my first point you think you might want to try the series' then I warn you, spoilers ahead. None of the three alien species that we encounter in the TFL:BTF series can be described as thinking like humans. One of the main characters introduces a riddle when they are first talking about one of the alien species they encounter that exemplifies how alien the aliens are to humans. The riddle is this. "Feathers or Lead." (to give context, the story told is that you have to pick one of the two, however it is a devil giving you the choice and the devil is constantly changing his mind as to which answer is the right one). The point made is that, without being the thing giving you the choice you have no way of thinking like the thing that is giving you the choice. And that even if you can approximate it using human ideas and emotions it is still unlikely that you will ever be able to fully understand the thing and the things context behind the things choice.

...I don't know how well I've explained that.

Novastorme.

Which is to say that, physiologically, you don't have to make the alien that different for it to feel alien. But if they have alien psychology... that's alien. Even if they look human. Especially because so often we end up with humans in rubber suits. Even if they're renderered in a more impressive way, that's all they really are, because they're still psychologically human. See: most fantasy races, most sci-fi races, etc.

The race I've seen that feels most alien to me, to this day, is probably the Soft Ones in Asimov's The Gods Themselves. For one thing, this is a race with three sexes. And Homestuck fans, put your hands down, Trolls are monosex, and have four relationship roles. The Soft Ones actually have three biological sexes. Not only that, but they're soft... or rather, immaterial. They can pass through things. Reproduction occurs by passing inside of each other.

And of course the society is also alien. Not as alien as it could be (partly because this was the 70s and partly because writing truly alien aliens wasn't the point of the story). And then there's The Big Twist, which really turns everything we know about the Soft Ones inside out. But I don't want to spoil it.

Honestly, they're still not that alien. Big parts of the human psyche remain. But I think that just shows how close to human models we stick: that I can see something still kinda human as waay more alien then most things.

I'm a big bloody fan of that story concept honestly! I think it might make for better short story territory--it could be told in the exact kind of condensed chunk short stories are good for to get the meaning across, and I'm not really sure sort of, uh, how the filming would work. Sort of, cutting between the POVs but not explaining it, kinda hiding it? Iunno. I'm a writerman and know nothing of moving images. I also just really like short stories.
Although, those are also some pretty powerful images to see actually filmed in action.

My own personal writing project and worldbuilding is centred around this stuff. I think it's somewhat understandable that most fiction is pretty anthropocentric, what with being written by humans, for humans, but I've always been interested in--at least trying to--shatter that human-centric feeling. It makes complete logical sense, though, still, I always do feel a bit disappointed when every RPG puts humans as the default and boring 'choose +2 points where you want' race. What other species would be the default, after all?

I think a couple of great examples I'm personally very fond of are in Slaughterhouse-Five, though it is played a lot more farcical and satirical than in a real sense of worldbuilding. The Tralfamadorian perception of a human is actually kind of similar to the way your fella sees a human--as a long centipede kind of form, with a baby at one end and a dead old man at the other. The reason being that they of course perceive all moments of time at once.

The webcomic-ey sorta thing (It's in that Homestuck tradition of being a bit of a multimedia mess) Awful Hospital also looked brilliantly at this.
A big part of its worldbuilding is the sentience of everything, and how the universe is composed of the opposing perspectives of all things. It's described as like: on earth, we have a world where humans are born and make cars and drive them. At the exact same time, in the same space, there is a world where sentient cars are born in factories and are gifted central nervous system-creatures to help pilot them around until their death. Neither of these perspectives are wrong: they both coexist and are brought into being by being perceived.
So on.
It makes a point, even, that when a group of humans form into a crowd--that crowd is, for a brief moment, a singular, sentient creature, that dies immediately when that crowd disperses.

I've been working on the legal, encyclopedic definition of a human being from the context of this worldbuilding. This is what I've figured so far:
3 Compressional Involuntarily Drifting Oxygen-Respiring Tubiformous Carbonoform

'3 Compressional Involuntary Drifting' refers to the human experience of linear time--involuntarily moving through three 'compressions' (a sort of combined measurement of space-time, which are essentially the same thing) at a time. As opposed to Compressional Sweeping where something simultaneously exists in all these compressions at once, or Compressional Hopping, where something just sort of pops between them.
Oxygen-respiring is obvious, tubiformous refers to the fact that a human is basically a big tube, morphologically not really distinct from a worm other than a bunch of added floppy bits, and carbonoform refers to, uh, us being carbon.

I think it's a bit clunky, I don't know. I'd be interested in seeing everyone else's thoughts on a sort of, bored bureaucratic alien definition of a human.