You might get a faction gain/loss for killing that mob or handing in a quest which will have consequences for multiple factions but I always thought mobs are on a specific faction.
For example:-
http://wiki.project1999.com/Banker_Willaen
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Of most bankers this is one of the most annoying to do if you're faction grinding (maybe not *the* most annoying).
He's on Kelethin Merchants which isn't something you can just raise by killing orcs etc, you have to do 1 of 2 annoying quests definitely hundreds if not 1000+ times to faction up. These quests faction up more than Kelethin Merchants but that's the one that'll count for him ^^