Not sure there's a horse blanket, but Cooler has a lot of Native American stuff over at 3DCommune. Search for Cooler in their free stuff and you should get it.

If you have Poser 5 or 6, you could use the cloth room to drape a cloth square over the horse and then paint an appropriate texture map for it

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You just can't put the words "Poserites" and "happy" in the same sentence - didn't you know that? LaurieA
  Using Poser since 2002. Currently at Version 11.1 - Win 10.