Alternative Armies (www.alternative-armies.com) have some 15mm unridden horses.
Museum Miniatures (www.museumminiatures.co.uk) have some in their equipment section under farm animals.
Essex Miniatures (www.essexminiatures.co.uk) do a “pick your own” horse pack in 15mm if you can find a suitable one.
Donnington (/shop.ancient-modern.co.uk) do a draught horse in yoke that might do at a pinch?
Peter Pig (www.peterpig.co.uk) do a Plains Indian pack of 2 horse holders and 6 horses.
Hope that helps.
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