Hi,

It could very well be a microchip. That's about the correct spot for one; on the left side halfway between the poll and the withers in the Neuchal ligament just below the mane. Hard to tell which chip manufacturer it might be though. Ask your veterinarian if he/she has a "universal"  microchip scanner. That will read all the different chips presently used in the animal trade. It'll just read a number. It will not tell you anything else. And there's no central registry either. Each manufacturer usually operates their own registry. But once you have that number keep it part of your horse's records. It will serve as an invisible tattoo, positive ID for the rest of her life. ~FH