Green’s Dictionary of Slang

1. an over-made-up or over-dressed woman.

1912		Motion Picture News 6 25: [He] says that he has the lady all picked out for him, all dressed up, like Astor’s pet cow, and that to-day she will come for inspection.
1921		Eve. Star (Wash., DC) 2 Oct. 3/4: How any man could waste two valuable hours a day dressing himself like Mr Astor’s pet horse is too deep for me.
1922		Bisbee Dly Rev. 29 Oct. 12/5: Usually when you are dressed up like Mrs Astor’s pet horse the part doesn’t give you any chance to act.
1935		S. Walker Mrs Astor’s Horse 8: [T]he gilded, over-stuffed magnificence of Mrs. Astor led the peasants to say of one who was rather ostentatiously dolled up: ‘She is dressed up like Mrs. Astor’s plush horse.’ Sometimes the phrase was ‘Mrs. Astor’s pet horse’.
c.1935		‘Mae West in “The Hip Flipper”’ [comic strip] in B. Adelman Tijuana Bibles (1997) 98: [He] spent enough of the old ‘geetus’ [...] in decking out Lotta like Astor’s pet poodle.
1948		J.G. Cozzens Guard of Honor 269: ‘He said I wasn’t to say that; it was unrefined. That’s why I’m dressed up like Mrs. Astor’s horse’.
1950, 1967–9		in DARE.
1996		(con. 1931) A.G. Marsala in Mississippi Rag Apr. n.p.: I opened with the band at the Edgewater Beach Hotel and, wouldn’t you know, suffered a pratfall the first night. Mother helped me dress, did my hair, and fussed over me as if I were Mrs. Astor’s pet horse.
2000		theStranger.com 10:5 19–25 Oct. 🌐 Viewpoints progress from the most innocent to the most corrupt, preserving slang from the era (1939) that ought to be immortalized: ‘like Astor’s pet horse,’ ‘butter wouldn’t melt in his mouth,’ ‘patch on a man’s ass.’.

2. an arrogant, haughty person.

1967–9		in DARE.
1998		Boston Phoenix 6 Apr. 🌐 Most of these folks are just working folks. You don’t get Lady Astor’s pet horse.