Mrs. Astor's pet horse

Mrs. Astor's pet horse

old-fashioned Someone dressed or decorated very ostentatiously; someone who is particularly pretentious or showy in appearance. The "Mrs. Astor" in the phrase refers to American financier William Astor's wife Caroline, a legendary 19th-century New York City socialite known for her lavish parties. Look at her swanning about in all her furs and jewelry, like Mrs. Astor's pet horse.
See also: horse, pet