As mentioned, there must have been something particularly distinctive about the horses for them to have been immediately identified as "post".

The only thing that comes to mind is something about shaving the hair in the horse. Usually, on what would be the "hip" (croup? - I'm equinely challenged). That is, the hair is thinned enough that the eye perceives letters or shapes.
I believe the Royal Canadian Mounted Police today has "RCMP" on their mounts. Perhaps some kind of lettering or design was what led the Bennets to that conclusion.

This would create a readily identifiable (and harmless) way of setting a post horse apart. For an owner, it makes them easy to pick out yours, it makes them harder to steal, and could even be a sort of advertisement.