if you resort to calling yourself "sadkin" or "gloomkin" then you're just resorting to extreme measures to be a special snowflake
Most otherkin I've met and talked to are younger adolescents. I think it's less of them actually believing they're a galaxy or a dog, and more of them trying to make an identity up because they don't like what their current identity is, and aren't emotionally stable enough to really do something about it.
It's tragic in a cringy way; instead of trying to figure out the cause for your anxiety or depression, you just justify it by saying it's just one of your core characteristics. Same goes for other character flaws like aggression, lack of attentiveness and even low self-esteem