Hi Mike,
Hmmmm. I wonder if some previous owner was soldering nearby the corral and the tip touched the corral. The plastic reaction issue is still a possibility, but with a culprit other than the horses themselves. (Magic Rub Prismcolor white erasers also react with plastic with which they come in contact. Back in the days of VCR time-shifting, I used to keep an eraser handy to erase the labels when I reused a VHS tape for a new program. One time I left the eraser sitting on top of a cassette for what turned out to be a few weeks, and the eraser actually melted the plastic and "ate" its way right into the cassette.)
As for a repair, other than some localized filing of the high spots and filling in of the low spots with something like Squadron Green putty, then sanding and repainting the corral, I have no simple solution. Of course, that, and probably any other "repair" approach, would render the corral no longer "original" from a collectors' standpoint.
Good luck, Mike.
Chris