"A Willing Horse" is an idiom originated from animal's behaviour as it is essential to note that there are some idiom from animals and animals' behaviours.
"A willing horse" as an idiom is indicating an attitude of being working hard with honest and sincerity without complaining.
A man or woman who works hard without giving any complaint especially when having what to show for the work can be contemplated as
a willing horse.
Examples:
♣ Mr. Smith is one of the willing
horses of the company through whom the company is attaining progress.