Think of a word to describe a single animal. Change the third letter to get a word that describes the plural of that animal. Both are nouns, and neither word contains an "s."The solution is:
HEAD, HERD, as in: The farmer had 123 head of cattle in his herd.I sent this in as the obvious intended answer, but it makes no sense to me.
The word "head" in that sentence refers to all 123 animals, not a single animal. The clue is wrong.
It can be used for the singular, such as:
Owners are charged a per diem for each head of livestock pastured.Yes, but i could also say I sold each sheep for $100, and then sheep would refer to one animal in that sentence. But the word "sheep" is still a collective noun that can refer to any number of animals.