In the UK the term "good doer" is used.
An easy keeper is a horse that doesn't seem to need much food to maintain good weight. It's often mentioned in sales ads as a good thing - the horse is cheap to keep.
However, an "easy keeper" is more likely to get fat and develop obesity-related diseases. They may need to have their grazing restricted and extra work to keep their weight where it belongs.
Fjord horses, like this mare, are often "easy keepers."