Its been explained on other GTA forums that the only solutions are:
2 Player Mode (Proved fake as there is only 1 health bar and 1 weapon HUD at the top right where CJ'S health is)
Pedestrian ( I think/know it cannot be a pedestrian because Pedestrians are never seen in the wooded areas and Pedestrians do not disappear)
Cop ( same explanation as Pedestrian, they do not disappear and do not travel in wooded areas)
Prostitute ( Again, they do not travel in wooded areas and do not disappear)
So the chances are that this is real footage but i was looking on a forum about GTA: San Andreas and they said it could of been faked using an animation program but i think they said something about the "thing" disapearing in 3 frames and not 1 so that evidence of being faked is fake.