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counter for large animals (e.g. head of cattle); counter for insects in a collection; counter for helmets, masks, etc.
「頭」 has other readings with different meanings.
牛が三頭いる牧場だ。
It's a ranch with three cows.
動物園に象が二頭やって来た。
Two elephants came to the zoo.
この農場では百頭以上の馬が飼われている。
Over a hundred horses are kept on this farm.