Became aware of this after visiting the Mütter Museum in Philadelphia-
It's really an interesting place to visit if you are ever in the area.
Or maybe Harry had implant modification done where small pieces of 'teflon, coral or silicone are inserted beneath the skin of the forehead, giving the appearance of small horns. These can be gradually replaced with larger and larger pieces as the skin stretches, creating larger horns. Coral is no longer used for larger horns, as it tends to be too brittle.' (Wikipedia)
Like the Enigma did-