A hard keeper or poor doer is, of course, the opposite of an easy keeper - a horse that has a fast metabolism and loses weight easily.
Such horses may need extra or even special feed to keep weight on. Sugar beet pulp is a popular feed in Europe for "poor doers" - this is what's left of sugar beet after the sugar has been extracted and refined.
Older horses often become hard keepers, especially as their teeth become bad.
Thoroughbreds, like this racehorse being walked out on the track, tend to be hard keepers.