From: Andrael	

By the way, it's cats Ranma's afraid of, not foxes; 

and after going up against a giant eight-headed hentai serpent and a 
giant winged minotaur with squid tentacles (among other things), I don't 
think he'd be fazed much by Kurama's little fox ears.<<

That reminds me, foxes are from the same branch as dogs not cats. So Ranma is definitely unaffected by Kurama's Youhko form.