This is appallingly spoilery; be warned.
The Harry-is-a-Horcrux theory seems to have run around fandom like wildfire. I admit it explains the Parseltongue and the mental link, but I note that it requires:
- Horcruxes can be made accidentially (which, I suppose, if the spell is to be cast at the instant of a murder, might work);
- Horcruxes can be made without the caster's knowledge (which, I suppose, if you're promptly disembodied, might be understandable); and
- Horcruxes can not be recognized when the caster comes face-to-face with one (which, I suppose, might be implied by the fact that Voldemort doesn't feel the destruction of other Horcruxes).
This is pretty tough to pull off. But, if you can get past all that, note:
This means that Harry's not the sixth Horcrux, the unknown thing that's likely Gryffindor's or Ravenclaw's (with the seventh part residing in Voldemort's body). He's a seventh, one unknown to either side; and if he is, then he's in for a nasty surprise when he's destroyed the locket, the cup, the snake, the thing of Gryffindor or Ravenclaw's, and then faces Voldemort thinking that Voldemort is mortal.