well...i know...but i dont think that counts.the demon hunter unit you could use outside the campaign was a generic version of illidan, just like the death knights were generic arthas. i think the only demon hunter that had a demon form, lorewise,was illidan...but again, i could be wrong...it was a long time ago...every Demon Hunter beyond lvl 5 could use Metamorphosis... its their ultimate ability...
EDIT:yeah...i was, in fact, wrong. from wowwiki:
" As a demon hunter grows in power, they undergo a gradual evolution, becoming more and more demonic in appearance. The changes are cosmetic and can take many different forms, from burning eyes to black blood. By the time the demon hunter reaches the pinnacle of his development, he is a twisted and dark version of his former self - though the chaotic energy in his body does not affect his personality and allegiance (or so the demon hunters claim), it wreaks great changes on his physical form. The demon hunter’s gradual evolution achieves its pinnacle as they become a demonic entity. Their appearance has changed to something frightening: The exact changes vary, but can include glowing red eyes; horns; clawed feet and hands; nonfunctioning wings; cloven hooves; a spiked tail; dark skin; a deep, rumbling voice; and similar cosmetic changes. The demon hunter becomes an outsider. "