I learned 열심히 as diligently, though i suppose the adverb hard isbmuch more common
Be careful here - different dictionaries will give you different translations. Naver says: 열심히 hard, diligently, enthusiastically, (formal) zealously.
Why is this only singular "horse" and not plural "horses" if the hint for 말이 gives both? "The horses work hard" was wrong
if it were explicitly plural horses it would be:
말들이 열심히 일해요 the horses work hard
if it were making a general statement about horses
말은 열심히 일해요 horses work hard
but since its just "말이" you can tell its referring to a specific horse