There are many ancient varieties of this pun. This newer version was posted by Kelly Fitzpatrick in alt.callahans.

Today’s Pathetic Daily Snooze prominently features an advertisement showing the smiling mug of Bob Walker, a thirty year veteran of the horse slaughtering industry. Bob had never considered the possibilities of equine consumption until the recent controversy regarding the sale of mad-beef from the English countryside. Now, though, he is seen halfway ’round the globe, hoisting a lovely haunch of British ex-Racing Machine for all to admire.

The sign over his head reads “Cheval Butchers — the only one in the Midlands”. Not for long, one shudderingly suspects. I think I’ll continue to eat the questionable beef, instead, but then I’ve always been one to put the carnivore the horse.

Starshadow (alt.callahans) posted the following variant:

Back in the sixties there was a restroom in Seattle with the most literate graffiti in town. One of my favorites is, “I am, therefore I think — which is, of course, putting Descartes before the horse!”