Board Thread:Metal Gear Solid V/@comment-68.207.141.23-20140915183235/@comment-25366375-20140916064757

We know that the horn seems to grow in length throughout the game to further symbolises Big Boss' descent in to evil (demonic iconography), though I can't remember where I heard that. What we don't know is why or how. Or alternatively, it may just be different production stages, as we've seen some drastic changes in the appearance of Big Boss throughout the game's trailers and such. One theory I've heard is that the horn is just being pushed out by his body progressively, so that will either indiciate some time-skipping or we may get some other explanation entirely. Maybe the Diamond Dogs medical staff are trying to remove it in many different surgeries or something, I don't know, and we can't until release.

But Bluerock is entirely correct in saying that it's definitely not a cutaneous horn just based off the x-ray pictures. I don't want to make any definitive statements before the game's release, but I'm going to say it's doubtfully anything other than shrapnel. How Big Boss got it, we don't know (chopper crash features no such shrapnel FYI).