The "Monster in the Locker" (unofficial name) is a non-combative monster found in a room within the Otherworld version of Brookhaven Hospital in Silent Hill 3. Heather Mason finds this creature in the same room she finds the plastic bag in the garbage can.
As its name would imply, it is trapped inside a locker which, upon being opened, reveals that a layer of wire mesh stops it from escaping. The monster appears to be suspended by chains with hooks attached, as well as being held within a sack of some kind, obscuring its details from view. Regardless, it spends its time thrashing uselessly against its restraints and emitting gurgling noises as it does so, seemingly choking on blood constantly.
Its purpose or significance is unknown, as well as the reasons for its punishment, though it's likely that it is a sadistic warning to Heather as she travels the Otherworld. It is similar to the Prisoners and the Glutton, as they are both completely harmless and unable to fight. Unlike the Prisoner, this creature is impervious to any attack. There is also a strong possibility that this is Valtiel hanging upside down inside the locker, turning a valve hidden out of sight as he is usually seen doing.
The Monster in the Locker could point to Heather's childhood as Alessa Gillespie, immobile and helpless after the ritual immolation, without any relief from her pain. Nourishing God through her irrepressible hatred and despair to escape this vicious circle, Heather may unwillingly be being pushed into the role of her major caretaker, highly emphasized by Valtiel's presence.
Similarly, it could represent Heather, who feels trapped and hopeless in Silent Hill, feeling as if she had no choice but to travel down further into the painful Otherworld so she can avenge her father's death and destroy the God growing inside of her.
This could also be intended to warn the player about the possibility of receiving the Possessed Ending, which is determined by unleashing Heather's wrath through killing a crucially high amount of monsters.