Mexicans take English courses all their academic life (K-12). Among the handful of expressions and words that stick is "of course". Since no Mexican would consider fun such a dry expression, a rhyming companion is added: "my horse". It doesn't imply that the listener is a farm animal that needs to be ridden on the back. Not at all. It just rhymes. "Of course, my horse" is equivalent to "mos def".
Poncio: ¿Ya calaste al Instituto Mexicano del Sonido?
Pilatos: Of course, my horse. Esos broders si se rifan.
Poncio: Have you listened to the Mexican Institute of Sound?
Pilatos: Mos def. Those dudes are awesome.