The Wooden Pony is a devious device, designed for the slow sexual torture
of a beautiful woman. Its earliest use as a sado-sexual device was in
Japan, but a similar device (rail) was used in Europe as a general-purpose
torture. In it's most vicious form the Wooden Pony is a wide triangular
wooden plank, set sharp end up. The helpless woman is lifted into the air
and set upone the horse, her full body weight supported on her most tender
flesh. Her legs are forced wide by the width of the lower part of the
plank. Additional weights are attached to the ankles to keep her from
falling off, making her ordeal immediately unbearable.

Catholic Guilt's modifications include using a thin board, rather than a
triangular plank, to allow clearer filming of the labia as she rides the
Wooden Pony. The woman straddles it, tied loosely but well enough that she
can not dismount. Catholic Guilt's Wooden Pony is also lower to the ground,
just high enough to force slave to tiptoes to protect her tender flesh.
There is much helplessness as slave awaits the inevitable muscle fatigue.
Soon her calves tire and she uses the last of her strength to lower herself
as gently as possible to the hard, narrow surface of the Pony. Delicate
flesh is crushed between the Pony and her pelvis, laying the first of many
small bruises upon her sex. In moments her muscles can lift her and she
rises again, but only for a few minutes this time. Muscles throbbing, again
she lowers herself, carefully placing her weight on a new, unmarked place.
It takes longer to summon the strength to rise a third time, but after long
seconds riding the Wooden Pony she points her toes and pushes against
gravity, sweating from the exertion... As her flesh swells slave finds
comfort impossible and she squirms constantly. It is this constant motion
that gives "riding the Wooden Pony" its name.

Source: http://www.captivity.com/woodenpony/woodenpony.html