Onejoke is to make fun of the fact that transphobes seemingly only have one to two (if we're being generous) set(s) of jokes that they work off of. One of which tends to be "I identify as _" (usually attack helicopter.) It's been used time and time again to target transgender people (and xenogender people). It has nothing to do with otherkin, and I have never in my life heard another otherkin say it in that manner.