Quick bit of ISIS-ing, of the vaguely possible species, Detroit only seem to be listing candensis wapiti and White-lipped Deer, deer-wise. Of the two it'd have to be White-lipped, but they look a bit different to the ones I've seen. Maybe a bit longer of coat.
I should add that neither species is in the numers hown here, so either ISIS isn't up to date or this is both groups together! :d
Maybe hog deer...I remember the White-lipped Deer being somewhere else in the zoo, I think with the Przewalski's Horses. This exhibit was really big so some of the animals could've been spread out.