Boselaphus tragocamelus. I saw 2 keepers and a vet (I presume) with a dart gun as I walked along the fence. I was watching the herd bull, so I didn't see the dart being fired. I was expecting the target animal to go down - but it didn't. The keepers dispersed and I could not think why. It was only when I looked at this photo and saw this animal's little horns that I realised it is a young male: so I presume that the dart injected it with a testosterone inhibitor to prevent it from maturing into a 'blue bull'.
@gentle lemur The look on its face says 'what the hell did you do that for...' I presume the dart just falls out and they retrieve it when(if) they find it.