The Bulthar Herdleader looks like an Ulthork
 

The Bulthar Herdleader is supposed to look like this:

http://wiki.project1999.com/A_Bulthar_Herdleader

But he looks like an Ulthork in game.

I see him in game exactly as that image.

I casted Ignite / Chill bones on him as i ran through to see how resistant he was last night, dunno if that the same one u seeing but it changed hi to an ulthrok

Sounds like this would be the case. Similar to Cazel in Oasis being cast on using a spell that alters his graphic. When it wears off he default to a cyclops :)

velks spiders revert to the brown desert type graphic after being skeleised also