The word is an adjective, adverb, conjunction and a pronoun. Technically it refers to two items or people so is a "dual pronoun" when used thus. Otherwise, it is not a plural.
"You" can be both singular and plural.
It is both a singular or plural noun.
Yes, "sheep" can be used as both the singular and plural form.
The indefinite pronoun 'both' is a plural form.Example: Both John and Kevin were going by bus.
Flesh is both singular and plural
Rabies. It is both the singular and the plural.
Parsley is both singular and plural
The plural possessive is both companies'. example: Both companies' contracts will be reviewed at the meeting.
Sheep is both singular and plural; the plural of foot is feet.
Seals (as the plural for both the animal, and of a symbol)
Mumps is plural. But it is both singular and plural is construction.
The term "gulf" is singular. It refers to a large area of sea or ocean that is partly enclosed by land. If referring to multiple gulfs, it is correct to use the plural form "gulfs."