https://youtu.be/jB1XvMyEBNA?t=2m14s
I mean is it 
? I mean, he is dual wielding a weapon, but its not Axes and he seems very samurai like. So who is this mystery person or is he just some random guy?
I posted a theory comment on the videos page. I assume its a corrupted form of Shen. Possibly the rival team that opposes the PROJECT team. Maybe the PROJECT team were created to destroy this force that Yi keeps seeing in his head? its just a theory.
Well, in the Noxian Invasion, Yi's village was bombarded by Singed's chemicals. That's probably what those visions are right before, as seen when Yi sees his village and begins to panic. The horned warrior would, likely, be a form of Noxian soldier. Likely with thicker armor to resist the chemicals. However, there is the weapons he had that could change that. They are very long thin swords that match the theme of Ionia. Could it be an Ionian warrior maddened by Singed's chemicals? Or is it deeper than that? Could the horned warrior represent Yi's subconscious guilt that he could not save his village, justified by the fact that it came to him when he was captured meaning he feels like he is caged by his guilt and cannot overcome it?
Everything said here is just a theory and speculation, we must wait riot to tell us the official lore of that. Wait and see :D
