This is English, not Latin, grammar.
As I said, unfortunately "sheep" can be singular or plural.
"The first sheep is in the fold, but the other sheep are in the field."
A very interesting book: "Grammatica Linguae Anglicanae" (Grammar of the English Language) came out in six editions (last in 1765, posthumously) by the mathematician John Wallis, 1616-1703, an important early member of the Royal Society). There is a modern edition and translation by J A Kemp published by Longmans in 1972.
He was one of the first to emphasize that Latin grammar is not useful to understand English (declension of noun cases is not a thing).
As befits a scholar of his era, it was published in Latin, just as Newton and the other Royal Society members did.