So--Henri de Toulouse-Lautrec's Equestrienne (At the Cirque Fernando), 1887–88. More familiarly "the great big painting of the massive circus horse's ass."
In June of 1983 I was at a cousin's graduation from the Bronx High School of Science. E.L. Doctorow was the speaker. Doctorow had recently visited the Italian city of Arezzo and visited Piero della Francesca's masterwork fresco cycle: the Legend of the True Cross. That's another painting with a famous horse's bottom, sacred, really, to Toulouse-Lautrec's prophane:
A big part of Doctorow's lecture... it was about innocent horseflesh. Another part... it was about human suffering. Christ, after all--in another near part of the painting, he's getting it, not far from that great calm white muscular bottom.
In any case, these are paintings to love, and to contemplate, and just getting to a museum like the Art Institute of Chicago--for me, it's one of life's treats.