As has been lamented here and elsewhere, the majority of San Diego's hoofstock collection has dwindled in the past few years, either through attrition or through transfers to private holders.
Both the royal antelope and rhebok were imported by San Diego with the hope of creating a sustainable population that could be loaned out to other zoos (probably while retaining ownership ... see red river hogs and koalas as examples). Over the past decades they have been bringing in new hoofstock by the boatload, but an unfortunate number of them end up dwindling off. In the case of the two species mentioned above, I'm not sure if there are enough spaces for "tiny little antelope" or "plain gray gazelle thingies" in other zoos, especially with more established programs (e.g., blue duiker) trying to expand. (FYI, the exhibit the royal antelope are housed in used to be the Tasmanian devil enclosure, for those that remember back to then).
Other species which San Diego has imported (and then fizzled with) in recent history include bay duiker, Spanish ibex, blue sheep, forest buffalo, Persian fallow deer ... and the list goes on.
A few have done reasonably well, notably the steenbok, Japanese serow, goral, and the addition of new blood to the red-flanked duiker population.
The breeding male suni died in 2004, leaving them with an all-female group which have slowly aged and died off. The last few were sent to a private facility in Florida ... I'm not sure if it was for "breeding" or just to clear up space. When I last saw them in 2006, they were acknowledged as being "functionally extinct" in North America ...
