Okay this is entirely because I just have some Thoughts on the nonhuman nonbinary trope. Basically, I've seen some discussion of it before, and the main thing that discussion and criticism of it focused on was basically the implicit dehumanization of nonbinary people -- that they must be robots, or aliens, or shapeshifters or whatever. But honestly, far more than that, I've always found the worse half of the trope to be the converse side of it: the concept that only robots/aliens/shapeshifters are nonbinary.
While the trope exists for many reasons, and there are always examples clearly not intended this way, the trope as a whole often revolves around making a nonbinary gender palatable to a transphobic audience, and it does so by basically upholding a very basic transphobic concept: the idea that your gender is determined by your physical body you were born with.
The nonhuman nonbinary trope is a trope of excuses and appeasement. It's a trope of justification: they're nonbinary, because X. It's a trope of creating special exceptions that transphobes can accept: that this character isn't a man or a woman, but they're not physically a male or female human, so it's okay! It's not human, and therefore doesn't have to follow the same rules as humans, so there's no need to challenge your worldview all that much. That shapeshifter has whatever parts it wants, the robot doesn't have any, the alien has something else entirely. Your solid, immutably sex-locked barriers between the genders are unharmed, because this character was never placed behind them to begin with.
In essense, it doesn't just dehumanize nonbinary people, but blatantly ignores that most of us who exist in real life are trans.
And the part of that trope that hurt me the most, I think, was how easy it was to keep thinking in those terms. To still mentally categorize nonbinary as a special category for the robots and aliens and shapeshifters, but never a concept that could apply to a normal, real human.
Even a few years ago when I knew on some level that regular-ass nonbinary humans existed, it was still this deeply other category -- something that wasn't really applicable to day-to-day life, because on an unconscious level, it was still just a thing for fiction. I think a lot of "supportive" people out there still think that way, too. The people who say cutesy stuff like "boy, girl, tentacle alien from outer space" and stuff like that -- I wonder, how much of that is shaped by simply not having a cultural image and context of a nonbinary character who is allowed to be a human? Not merely the dehumanization, but the discomfort of acknowledging that nonbinary humans exist, and by implication, might have bodies that aren't perfectly, inhumanly* androgynous?
If I hadn't gotten into the online spaces I had, I wonder how much longer it would have taken me to figure things out. As it is, the fandom that got me thinking about gender was actually Undertale, and I think it was because it was the first fandom where I saw depictions of (usually trans!) nonbinary gender for actual, natural human beings. I'd watched Steven Universe and seen Stevonnie by then, but they are ultimately just another case of this trope. They're a well-intentioned one, definitely, and still extremely valuable for their audience -- this is the kind of thing I mean, when I say some examples are not intended this was at all, because sometimes you just wanna be a genderless space alien, plus it's useful to have this trope for Baby's First Exposure To Nonbinary Characters or Can We Get Literally Any Rep At All, My Crops Are Dying, but they're ultimately one that can still be dismissed, because after all, they're nonbinary because they're fusion of a boy and a girl, and therefore still exist in the fantasy-land of this hypothetical perfect cis enby that very rarely, if ever, exists.
It was in the Undertale fandom that I first got exposed, via fanfic, to stories about characters who are firmly human and still nonbinary. Characters who may have an assigned binary sex, but reject it, and consider themselves neither. Characters who sometimes even dealt, in those stories, with issues surrounding gender, or were acknowledged as having not always been known by the name or pronouns they now used, or were just androgynous because they wanted to be.
That was the game-changer for me. I know that sounds like a dumb place to have figured it out, but that was when my feelings on the matter started to shift. Prior to realizing I was maybe nonbinary, I had a whole-ass phase of being really fond of Frisk and Chara (and later Kris), and also drawing a lot of designs for androgynous/nonbinary characters (at this point rationalizing it as "I just think they're neat".)
I'll admit there's a decent chance of correlation/causation issues here. The Undertale fandom also has more "don't misgender these characters" type stuff going on, because Frisk is the main character, whereas Stevonnie is most a minor role, and I also didn't have a lot of other nonbinary characters to refer to at that time. Maybe I would have imprinted on something totally different, if given the chance -- I know I have sort of now, although I have yet to outgrow the "can I just be frisk undertale" gender feelings, too. I know it's not an uncommon thing. But I also saw other characters before that -- the girl from Ouran Host Club, BMO, Jakotsu from Inuyasha (I was often rather fond of the androgynous/feminine male characters, now that I think about it), and maybe others I don't remember? But I was also less aware of gender then, so... it's not surprising that didn't awaken as much for me, I guess.
At this point I'm just getting kind of navel gazey, but my point I guess is that dehumanization isn't the only problem with having all your nonbinary characters be nonhuman. The other part is leaving people with the idea that those characters are only allowed to be nonbinary because they have the bodies to match, and that therefore nonbinary gender is a special category reserved only for certain nonhumans**, rather than something assigned-a-binary-gender-by-sex-at-birth humans can be, too!
*I am very much aware intersex people exist, and that some qualify as pretty darn androgynous. But I am referring not to natural human androgyny, but to the kind that comes from not being human at all.
**The rare examples of "hermaphrodite" characters are generally not interested in... y'know, actually portraying intersex humans in a remotely accurate way. They also often get written as only being that way for magic/sci-fi reasons, too, so bonus for erasing two categories of humans in one go!