Head can be a Plural Noun

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.