I've got some 15mm late Roman horses from Donnington which seem to have an unusual strapping both on their necks and thigh. I'm assuming this is leather. However I can't seem to find any historical visual reference to this strapping. I'd ask Donnington but they don't seem to be answering emails at the moment.
Sorry I'm not sure this post will show or attach a photo
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It's just multiple leather straps, usually studded. See the photos at http://www.comitatus.net/cavalryrecreate.html for instance, or this painting (https://i.pinimg.com/236x/4d/4a/3c/4d4a3cf9c71f41a3763fa99b59801200--pax-romana-roman-soldiers.jpg).
I'd say paint the lot in horse colour, so it fills the gaps between the strapping, and then paint individual straps in whatever colour you're using for the rest of the harnessing.
Thanks Duncan.
Martyn S