I haven't found a Poser character with a horse's head, but a number of people have created anthro figures by combining heads from Poser animals with Poser figures. Lemurtek did it with a number of animals, but hadn't done it with a horse.

Start a new post thread asking how to do it. PhilC is supposed to have a script that's supposed to make it an easy process, but I can't remember if it's only for human figures or if it's easy to use it combining animals with humans.

My visual indexes of Poser content are at http://www.sharecg.com/pf/rgagnon