Zero plurals are used for hunted animals (and sheep). People who hunt tigers might say 'I got two tiger yesterday'. No-one hunts zebras, do they?
It's possible that some people have extended the use from "any big animal bwana might shoot on safari" to "any other big animal bwana might see", but really I've never heard of zero plural for zebras, rhinoceroses, or hippopotamuses.
According to this site, which lists names for groups of nearly any animal or bird one can think of, it can be a herd, a zeal, or a dazzle.
At least among my friends, if someone mentioned "a dazzle of zebras" (or "a dazzle of zebra," I suppose) we would have no idea what he or she was talking about. (Or, given our ages, we'd probably think we heard wrong ... ) In normal conversation and prose, I'd stick with "herd."