Also the photo gallery has some pictures of other animals as well including.
Bison
Camels
Giraffes
Rhino
and some pictures of what would eventually become the Cheetah conservation station. In the photos for them it includes a photo of the maned wolves, the zebra when they were in the current oryx,gazelle exhibit and a type of antelope that I don't know what it is but I wish the zoo still had it.
Your mystery animal is either a Blesbok or a Bontebok. They look very similar and at that distance and with my limited knowledge I can't determine which one that is. Both are very lovely species, though.
Was there ever a brief moment when an Andean bear temporarily lived here? I have lived in the DC area my whole life (90s through today) and have a vague memory of seeing an Andean bear in an area where I didn't remember seeing them before and thinking, "I wonder why they moved it-- I liked the old exhibit better. Could it possibly have happened during American Trail construction or sometime when the sloth bears had recently left these grottos?