All of the horse figures currently available are commercial items.

In addition to the P4 Horse included in your Poser library, there's bloodsong's Heavy Horse and Wee Horsies, and odeathoflife's cartoon ponies (all here in the Marketplace), the riding horse at Imagination Works, PhilC's low-polygon horse (and a free cartoon donkey), and the DAZ Charger.

Cubed and DAZ have new horses in the works.