"nature is nonbinary"
every time i see the phrase "nature is nonbinary," my biologist heart SCREAMS. the word you're looking for is intersex, and intersexism(?)/intersex traits are incredibly overlooked!!!!!!!!
nonbinary refers to deviating from the gender binary, which is an entirely human invention. nothing in nature has a gender in the same way that humans do (similar to morality and ethics, which are also human inventions. for example: no, dolphins don't rape because they don't know or subscribe to the human notion of consent). intersex is an umbrella term that describes a variety of sex characteristics that deviate from the sex binary, including gonads and hormone levels to varying levels of hair and muscle mass (at least in humans).
let me go down the list of commonly attributed "nonbinary" plants and animals. (to preface: i'm well aware that hermaphrodite is considered a slur against intersex people, but i'm using it here in a purely biological sense, and for lack of a better term. also these are my opinion as a biology student and zoology nerd and intersex person. feel free to read more at the sources i link, there's plenty of nuance here!)
lots of fish have both ovarian and testicular tissues when they're born (according to Wikipedia my beloved), and one of the biggest examples is the clownfish. they are born as males and live in groups with one dominant female. when the female dies, the most dominant male transitions into a female. this is an example of sequential hermaphroditism. due to being born with both ovarian and testicular tissues, they are intersex.
fungi are a whole can of worms. Schizophyllum commune is a species that has an estimated 23,000+ mating types, and mating types are essentially the microorganism version of sexes. additionally, some fungi can sexually reproduce without needing a mating partner which is known as homothallism. this is similar to self-fertilization and self-pollination in plants, which requires an individual having both pollen (which carries sperm) and a carpel (which contains ovules that form eggs). i got a lil off track but anyways verdict: unclear but definitely not as simple as nonbinary or not.
now worms. many species of polychaetes, earthworms, and leeches are full hermaphrodites which means they produce both male and female gametes, or sperm and eggs. intersex stamp of approval
for some reason i've seen stick insects being mentioned and ??? there are distinct male and female stick insects, and some species have the ability to reproduce via parthenogenesis (asexual reproduction in which eggs don't need to be fertilized by sperm, typically resulting in identical offspring). this isn't nonbinary or intersex or anything, this is just an unusual reproductive strategy (by human standards) and is practiced by a lot of other animals including the brahminy blind snake <3
snails are a bit of yes and no. some freshwater snails are capable of self-fertilization, which doesn't count as intersex. BUT land snails do! most of them are hermaphroditic and can produce both sperm and eggs. to drive the point home, only some groups have distinct sexes. land snails are intersex!
another commonly cited example of "nature is nonbinary" is the seahorse, and is probably the closest we'll get to that statement. seahorses have distinct male and female sexes, and the one of the biggest differences between them is the male having a brood pouch to house eggs. the male takes on the role of carrying eggs until they hatch, which is really rare in the animal kingdom! it's a reversal of gender roles, but not necessarily nonbinary because it doesn't totally deviate from the male/female binary. it's not intersex either, because the brood pouch isn't analogous to a uterus. i could be swayed though, and there's an argument to be made that the brood pouch is a secondary sex characteristic, which is among the intersex "criteria."
reptiles in general are not usually brought up in the "nature is nonbinary" discussion but they're worth mentioning. sex determination is dependent on the temperature of the eggs. there's patterns IA, IB, and II. in pattern IA, eggs below a certain temperature will mostly be male, and eggs above a certain temperature will mostly be female. this is reversed in pattern IB. in pattern II, eggs at either extreme of temperature will mostly be female, and eggs at an intermediate temperature will mostly be male. there's a citation on wikipedia saying pattern II has resulted in intersex individuals but i can't confirm because the paper isn't open access. HOWEVER there's some evidence that some female snakes can have a hemipenes or can lack a hemipenes or hemiclitoris at all, but further research is required, including the proper classification of such organs. regardless, some snakes are undoubtedly intersex!
spotted hyenas live in matriarchal communities with females being more dominant than males, even as juveniles. adult females have a pseudo-penis (which is a huge clitoris and is capable of erection), a pseudo-scrotum (composed of fused labia), and no external vaginal opening. they give birth through the clitoris/pseudo-penis, which tears and takes a while to heal. the formation of the pseudo-penis is unrelated to the levels of androgen in the body (which is a category of hormone that's associated with male sex characteristics, and that includes testosterone). ultimately, the female spotted hyena is intersex.
if you've read this far, (omg thank you) then you might be wagging your finger at me like "well nonbinary is a human-made concept, but so are intersexism and hermaphroditism" and that's fair! but i'm using these words as a way to just categorize patterns in nature rather than as hard-and-fast "absolute truths" of nature. also nonbinary refers to gender, which is a deeply nuanced thing that we don't know for sure if animals subscribe to in the way that humans do. intersex is fluid in the same way gender is, but with different criteria and isn't as much of a social thing as gender is. sex isn't as concrete as XX and XY, but it's "more" concrete than gender in my opinion in that there are traits that correspond to specific genes and responses to stimuli.
all this to say: i'm a bit pissed at the complete ignorance about intersex conditions and traits, in favor of slapping the nonbinary label onto anything not stereotypically female and male. it feels like people forget that biology is so much more complex and interesting than your poor science teachers made it out to be. watch some documentaries, take an MOOC on coursera, read some nonfiction (audiobooks count <3). i'm not knocking the people who subscribe to the idea that "nature is nonbinary" but i'd love for some more intersex awareness and interest in biology, or even gender studies and anthropology and philosophy.