Neo/xenogenders are, in short, any gender that does not fit on the traditional, male to female /agender spectrum.
There are many reasons why one might identify with a Neogender.
They are most often used by neurodivergent, specifically autistic people. These people understand social constructs like gender very differently from their neurotypical/allistic counterparts
Many people think of their gender in a metaphorical way. You may identify as spacegender because you think of your gender as something vast and unknown. You might call yourself pupgender because your gender feels playful and cute like a puppy.
This can be hard for people who don’t experience gender this way to understand, but you don’t need to get it to respect it. These people are just as valid in their experience as any other trans person and deserve respect
Some people may consider their identity to be a form of self expression. Their gender(s) might be treated as one might treat their clothes or their bedroom, where they find what feels right, even if their understanding of gender is very different from what most might think of it as
These are similar to Neogenders in that they are, in short, non-traditional pronouns. It ranges from things like it/its, or xie/xir, that are similar to more traditional pronouns, to things like blue/blueself, cat/catself, space/spaceself, or whatever else you feel like.
It’s common to say that these are confusing or hard to use, and their users are usually patient with people who struggle or have ‘auxiliary pronouns’. These are pronouns that the average person would understand better.
It’s important to respect these pronouns and try your best to use them. If a person does not have auxiliary pronouns, it is still misgendering to not respect their pronouns, even if they don’t make sense to you, they are important to the user and you should try your best to use them.