I'd like to know more about the contexts in which they're found.
The Romans didn't have their own cavalry, they hired Germans or Celts for their cavalry. Given their distribution, I think there might be something to this. Even if they're decorative on the horse and the holes are where the hair went through to secure it. They didn't have saddles or stirrups.
Or maybe they were indicative of awards they won, like medals, which would be fastened to the horses.
Possibilities are endless, really. We need contexts.