A valid argument is one in which it's impossible to have the premises all true and the conclusion false. The central belief of Otherkin is that they are either spiritually or physically a creature from mythology. I would suggest that this is a false premise, therefore their belief would be invalid. So I cannot agree with you when you say that "their beliefs are no more or less valid than any other"--Kuro5hin user "BigChris"
Now that's what I call circular. "Their souls are human, so their souls are not inhuman. As their souls are not inhuman, they are human, proving that their souls are human."
(The article is at http://www.kuro5hin.org/story/2003/5/22/03514/1997 if anybody hasn't heard about it already. I've got a comment thread on it, but it got modded down to 1 after one vote by someone who, checking his comment rating history, has been busily searching through the thread and modding down anything pro-Otherkin and modding up anything anti-Otherkin, no matter what. I'd appreciate a few counter-votes; I'm getting zero reads because most people sort their views by rating instead of by date, a view where my post is near the top.)