The Probert Encyclopaedia of Nature

LIGHT HORSE

The term light horse refers to any breed of horse which is suitable for riding, and as such light horses and heavy horses overlap, with some breeds being suitable for riding and for draught work (pulling vehicles). Light horses have a lighter skeletal frame than heavy horses, well defined withers and a back that is not too broad. The first eight ribs tend to be somewhat flattened to allow the saddle to fit easily.
Research Light Horse