Relevance to Simon
This twitcher could represent the moment Simon got hit by the car, and the loud wail represents the pain he had during that moment and the screams he yells when he gets hurt.
For the fact that the upper walks on its hands and not its legs could represent how Simon can no longer use his own to walk.
The original sound was supposed to be an ambience sound that was to be heard (Yet less violent sounding and played much slower) the moment you walk into the facehead room, when you encounter multiple faceless monsters
Considering the fact that the Upper is the same as the normal slower, the animations are the same, explaining the reasoning behind the awkward animations when he attacks and the fact he only uses 1 leg to attack, the one that seems very crippled out of shape. The other leg (according to the bone structure) is the location of the head on the normal slower.
Despite the fact that the monster isn't supposed to move, if you do move far enough away, the animations are incorrectly timed, making it so the monster seems to be hand-walking oddly towards the player.