It would just be his ideal weight. A horse that keeps a good body weight with little food is called an 'easy keeper'.
wise horse
A horse that requires little extra dietary maintenance and maintains a healthy weight is called an easy keeper.
A horse like that would be called a hard keeper.
an easy keeper
An easy keeper
yes
Easy keeper
easy keeper
Easy Keeper
A horse
This type of horse is refered to as an "easy keeper".
a good doer