Ian has it right. It's the African wet nurse giving her milk to massa's baby while her own baby, lies unfed and unattended. All the pretty little horses refers to the wonderful things available to the owner's family that are not available to the African slaves.

I think this is essentially the same theme---and almost the same song---as Gershwin's "Summertime." Of course Gershwin's composition is softened for popular consumption.

      - Mark