I encountered variants of the 'otherkin' community in the mid/late 90s as a teenager. It was an interesting time. It existed in forums and chatrooms online at least that early. It wouldn't surprise me to find out there were BBSs for these communities even earlier than that.

Adolescents tend to engage in community-seeking behavior to a great extent, and if they're not finding it in their schools or local community, they'll almost certainly look to find it online.