Post riders or postriders describes a horse and rider postal delivery system that existed at various times and various places throughout history.

What was a post horse?

: a horse for use especially by couriers or mail carriers.

What does trocks mean?

Trock (uncountable) (fandom slang) A genre of music produced by fans of the British science-fiction television series Doctor Who, characterized by lyrics about its characters, settings, and plot elements.