I forgot about Suni being present on Zanzibar along with the duikers. I'm leaning towards that as the ID; Harvey's Duiker looks more reddish and with more black on the face than this individual has.
@sooty mangabey I didn't think much of it, to be honest. The only animals I recall seeing/have photos of were a few lizards and this antelope. It was 10,000 Tsh per person (about £3.50 / $4.30). It was also pouring rain when I visited, which was explained as the reason I didn't see some other reptiles.
As you noted, it was a side stop from seeing the red colobus in Jozani Forest.