I found the following expression in dictionary.
large herds of elephant and buffalo
A herd must be more than one, why using singular not plural?
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Elephant can be plural, like deer.
el·e·phant noun, often attributive \ˈe-lə-fənt\
plural elephants also elephant
Buffalo also can be plural, like deer (and elephant).
buf·fa·lo noun, often attributive \ˈbə-fə-ˌlō\
plural buf·fa·lo or buf·fa·loes also buf·fa·los
I think your dictionary is correct, and that more than one herd of elephants is "herds of elephant".
"Hippos fight for territory while herds of elephant, water buffalo and zebra depend on it for life." > http://www.bbc.co.uk/programmes/b01dq07f
"We begin with game viewing in the delightful Tarangire National Park, where herds of Elephant roam" > http://www.classicjourneys.co.uk/content.asp?id1=131
"...the largest herds of elephant in Africa." > http://www.safari-club.co.uk/botswana-safaris/
"see lions, leopards, herds of elephant, giraffe and much more" > http://www.hfholidays.co.uk/holidays/kenyan-safari
The reasoning is that you are indicating that there is more than one herd, rather that indicating that there is more than one elephant. Used in this way, with the focus on the herd, 'elephant' is the material the herd is composed of, so in the same way that we say cups of coffee or bars of chocolate, we say herds of elephant.
Because they are all of the type elephant
It's a herd of elephant amimals