Dobbin

A quiet, gentle horse used for ordinary riding or pulling carts, often one that's getting old or isn't particularly fast or spirited.

Possibly from 'Robert' or 'Robin' as a diminutive name for horses (like how 'Dobbin' became a generic horse name). The term became common in English by the 17th century, likely from Dutch or Germanic sources, and was often used affectionately for a common workhorse.