I was never a fan of FlashLight, but more of SombraLight.
One day, his shadowy self passes by Ponyville and only one pony can see him. He was in awe, but that didn't stop him. He hoped for this mare to help him. And thus, she accepted the request.
Time went by and he got used to haunting this mare. She got used to it, as well. They became good friends after a while, but that didn't stop him on his tracks.