Dominated units take the racial promotion of the civ that you are playing. For example as Svartalfar, I used dominate on Bannor units. However, the Bannor units, who are human, gained the elven promotion when I dominated them.
It doesn't make sense for a human unit to turn into an elven unit when an elven archmage (Svartalfar in this case) dominates them, and I'm pretty sure this isn't supposed to be happening. I'll check in WB to see if it happens with non-human units such as orcs maybe and whether the reverse can happen.
edit: just checked as the Clan, units that already have a racial promotion like the dwarves or elves, keep thiers, but human units turn orcish. I assume the same thing happens with a dwarven archmage. I'll check with a human archmage.
edit: ok, it looks like units with racial promotions keep thiers as they should when dominated by a human archmage. When dominated by an elven, orcish, or dwarven archmage, a human unit gains the racial promotion of the dominator. I know the clan can't get arcane units as that is closed to them, but I wanted to test what I was seeing here.
Patch e btw.