Mine has one, it's been there for the 9 years I have had her and never caused any problems, vet checks it when she does teeth and jabs but doesn't expect any issues with it.
My last horse had one. It looked like a small brown sphere in his eye partially attatched to the corpus nigra. It grew quite a bit over the 17 years I had him but never appeared to affect his eyesight although the vet told me it might. I believe they can be removed now via a simple procedure. If buying a horse with one I would want a vet to confirm it is just a cyst and not a tumour though.