The singular noun is donkey.
The plural noun is donkeys.
Examples:
My grandfather has a donkey at his farm. (singular)
The cart was pulled by two donkeys. (plural)
donkey's
The possessive plural of donkey is donkeys' whilst the singular possessive is donkey's.
donkey's
The singular possessive form is week's.
The singular possessive form is classmate's.
The possessive form is Amos's.
The singular possessive form for the noun gypsy is gypsy's.
The singular possessive form for country is county's.
The singular form for the plural noun countries is country; the singular possessive form is country's.
singular possessive: boy'splural possessive: boys'
The singular possessive form is speculum's.
The singular possessive form is aircraft's.