Slate's Daniel Engber delves into the mysterious world of animal masturbation:
Still, neither the fresh-sperm hypothesis nor its discredited cousin, the kamikaze-sperm hypothesis, can account for more than a small subset of animal masturbation. Reloading might explain the behavior of bucks, bulls, and male primates, all of which tend to ejaculate at the end of an autoerotic episode. But many other animals never reach that point. Horses rarely climax, despite masturbating dozens of times per dayso what motivates the dalliance of a stallion or, for that matter, a mare? Can evolution account for female masturbation in the animal kingdom? We don't really know.
And yes, there's a video slideshow. (I'm obliged to point out, per my contract, that a bull terrier makes an appearance.)