In
voodoo, one who is possessed ("ridden") by a
loa during a rite. The horse exhibits behavioral traits associated with the
spirit and, speaking with its voice, answers the questions and entreaties of the participants (or refuses to answer them -- the loa are tempermental sorts).
For further reading, check your local library for Zora Neale Hurston's Tell My Horse: Voodoo and Life in Haiti and Jamaica or Alfred Metraux's Voodoo In Haiti.
Papa Legba,
Come and open the gate.
Papa Legba,
To the city of camps.
Now, we're your children
Come and ride your horse
In the night
In the night
Come and ride your horse
-- Talking Heads