People who want to have sex with cartoon animals.
Not so disturbing as many Internet-spawned fetishes in and of itself, but the sheer mass of people who identify themselves as furries online is creepy as all hell. And, of course, gives its proponents license to claim that whacking off to Rescue Rangers is closer in the order of sexual deviances to being gay than, say, scat porn, and that they have some fundamental human right to flash their kinks around in public without getting reviled or laughed at.
And, eh. So what. Either it's a fad and he'll grow out of it whenever he stops hanging around with other furries and realizes he really isn't an otter or whatever, or he'll stick with it and tough ****. If you really want him to stop, get him away from the internet- people are always (consciously or not) inclined to act and think like those they are around, just cutting off his visits to the boards'll do it unless he's genuinely into the whole schtick, in which case just buy him a fursuit and wish him luck in life. You don't really need to find him new friends or anything, he'll do it himself once he can't retreat into virtual space anymore.
