“I may say this is a deluded generation, veiled with ignorance, that tho popery and slavery be riding in upon them, do not perceive it; tho I am sure there was no man born marked of God above another, for none comes into the world with a saddle on his back, neither any booted and spurred to ride him.”

Letter to Roger C. Weightman http://www.rjgeib.com/thoughts/jefferson/jefferson.html, declining to attend July 4th ceremonies in Washington D.C. celebrating the 50th anniversary of Independence, because of his health. This was Jefferson's last letter http://www.rjgeib.com/thoughts/jefferson/jefferson.html. (24 June 1826)
1820s
Context: All eyes are opened, or opening, to the rights of man. The general spread of the light of science has already laid open to every view the palpable truth, that the mass of mankind has not been born with saddles on their backs, nor a favored few booted and spurred, ready to ride them legitimately, by the grace of God. These are grounds of hope for others. For ourselves, let the annual return of this day forever refresh our recollections of these rights, and an undiminished devotion to them.

Source: First Things, Last Things (1971), p. 65

Letter to Lord Godolphin (12 September 1707), from Edward Gregg, Queen Anne (Yale University Press, 2001), p. 250.

The Philosophical Emperor, a Political Experiment, or, The Progress of a False Position: (1841)

The Story of Kin Wen and the Miraculous Tusk
Kai Lung Unrolls His Mat (1928)

Sudden Death (1983)
Variant: "If the World Made Sense, Men Would Ride Sidesaddle" was the title of a 1993 one-man comedy by Ed Navis, performed at Wings Theatre, New York.
Variant: If the world were a logical place, then men would ride side-saddle.

Ode, inscribed to W. H. Channing
Bartlett's Familiar Quotations, 10th ed. (1919)

Setzen wir Deutschland, so zu sagen, in den Sattel! Reiten wird es schon können.
Speech to Parliament of Confederation (1867)
1860s

Book I, Ch. 7.
The Life and Opinions of Tristram Shandy, Gentleman (1760-1767)