✘ your demon's name is yours

[yardbird]

DISCLAIMER!
Like with any of the Silent Hill games, the monsters are usually interpreted with symbolic significance that is specific to a particular character.
I do not claim that any of these are canon, as they are my own personal interpretation in further analysis to Murphy's personality. Since most of this can be considered "headcanon", it's separate from the application.
It all boils down to the fact that there are (usually) no right or wrong answers here -- because the overall meaning is generally left up to what the player gets out of the story. Thus is the beauty of Silent Hill, really.

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MONSTERS
PRISONERS
[ Reference; 1, 2 ]
The Prisoners are one of the most obvious symbolism. These guys come in two different forms (the larger ones are called "Juggernauts" because they are larger and more deadly), though both sport clown-like grins on their faces. They clearly are manifestations of the kinds of people Murphy had been incarcerated with. Murphy not only saw the worst in himself, but the worst in humanity in itself for several years. For him, humans are monstrous and therefore it makes more sense that the monsters of Silent Hill are more humanoid to him than the predecessors who had encountered the town.
Beyond that, do I seriously have to go into this one in-depth?
...I didn't think so.

SCREAMERS
[ Reference; 1 ]
It's believed that the Screamers are twisted versions of Murphy's ex-wife, Carol.
Some time before or after Murphy was arrested, Carol had sent him a letter telling him that she never wanted to speak to him again. She blamed Murphy for Charlie's death, and couldn't look at him because of it.
When the Screamers are first introduced, one is being brutally beaten by one of the escaped stereotypes convicts, Sanchez. At first, Murphy heard the pitiful cries of a victim, but when he realized that she was a monster, it was too late. The Screamer lets out an agonizing cry that momentarily incapacitates Murphy, and then attempts to ruthlessly slash at him with her long fingernails. This can be interpreted as the pain Murphy felt when Carol refused to forgive him for letting her and Charlie down. Because damn, the woman really hit him where it hurt most.

DOLLS
[ Reference; 1, 2 ]
The Dolls seemed to symbolize a certain kind of feminine objectivity, at first appearing as passive, inanimate figures. Murphy even mistakes one for being a sex doll.
It's just their shadows that you really have to watch out for... While the dolls can be first introduced drawing you in by their tortured sobbings, the shadows (visible only with the help of an ultraviolet light) will come out and attack. Just like how Murphy avoided the loss of his son by seeking vengeance instead, he is then distracted by the real danger and is attacked violently by the shades as a result.
The sexual nature of the dolls' appearances could also represent a specific deprivation, since Murphy had been in prison for such a long time. Because come on, their original placeholder name had been "Flirt".

CORPSES...
[ Reference; 1, 2 ]
The Wall Corpse/Trapped Souls are guys that come in two forms as well. They are either submerged in walls, or they're trapped in shelf-like cages. They appear to be rotten cadavers that are heavily misshapen and mutilated. The corpses trapped in the wall will spew acid blood that will knock Murphy off his feet if he isn't careful when treading across these things. It can be assumed that these are concepts of the people who were hurt and/or killed due to Murphy's determination to kill Napier.

VOID
[ Reference; 1 ]
As the name implies, the Void is a warped portal that chases after Murphy and gradually tries to rip his skin clean off. It deteriorates any living thing that it touches, and Murphy will be erased from existence if he isn't fast enough. The Void cannot be killed, and the only way to survive it is to book it like a bat out of Hell.
Sounds nice, huh?
When Murphy first encounters this force, he remembers the voice of Frank Coleridge telling him to run. Later, the same voice tells him, "You can't outrun yourself, Murphy." Implying that perhaps the Void is a representation of the guilt Murphy is running from but unable to own up to. The only way to slow it down is to throw Trapped Souls down in its path, much like how Murphy endangered so many people just to get himself arrested in the first place.

WEEPING BATS
[ Reference; 1 ]
Weeping Bats are tall and lanky creatures and they are silly. They have a silly walk. Murphy probably has a silly walk, too. So they might be representations of Murphy's silly backwards walk that haunts him.
Actually, not really.
But in spite of their tall and gangly limbs, they keep their arms constrained. It's really confirmed that they're a representation of Murphy's time in solitary confinement, which he had experienced while in prison (most likely after the incident with Frank Coleridge). It's also worth noting that solitary confinement is an act of imprisonment isolating a prisoner from all human contact, with the exception of staff. It is used when taking extra measures of protection as well as suicide watch, which may have been an issue with Murphy after Napier and Coleridge were killed.

WHEELMAN
[ Reference; 1 ]
The Wheelman is Frank Coleridge. While he had survived Sewell pummeling his face in, Frank might as well have been dead, now rendered a wheelchair-bound vegetable that suffers a slow and agonizing death.
The Wheelman follows Murphy since his first venture into Silent Hill, seemingly directing him on the right general path through town, just as Frank had tried to do for him while he was alive. Sometimes, when following the trails left behind by the Wheelman, reflections of Frank Coleridge can be seen in one of the few mirrors left in tact in town, further connecting Coleridge's role as the Wheelman.
It initially serves as a guide for Murphy, before the end in which Murphy is forced to confront what happened to Coleridge. Additionally, the only way to kill the Wheelman at the end is to cut off the life support tanks sustaining it.

BOGEYMAN
[ Reference; 1 ]
The Bogeyman is completely subjective to the individual, it seems. To Murphy, it's a monster, and reveals itself to be Napier to him. To Anne, it's Murphy himself. Both Anne and Murphy have lost loved ones to their own literal "Bogeyman" -- Murphy lost his son to Napier, and Anne's father wound up being collateral damage as a result of Murphy's vengeance path. We oftentimes imagine the Bogeyman as being something that we fear and/or hate the most. To Anne and Murphy, the Bogeyman is seen as the monster that destroyed their lives.
There is also a heavy theme on the death of children, and the Bogeyman plays a big role in that. Not only is it apparently warded off by a child's rhyme, but the only ones we truly see it kill on screen is a little boy (who Murphy sees as his son), and "Napier" (a pedophile). The scene in which the Bogeyman is shown killing Napier is an obvious representation of Murphy killing his son's murderer.



war war war, war against your soul

OTHERWORLD
Like with the monsters, the nightmarish manifestation of Silent Hill, also known as the Otherworld, also represents a specific significance to an individual. In this case, Murphy is experiencing his own personal Hell, as it greatly represents a prison most of the time.

Murphy's Otherworld also consists of industrial clockwork, pulsing torn skins and tubes, as well as Escher-like staircases and rooms, which are sometimes flipped upside-down in order to give a more chaotic implication of Murphy's life that has long since run out of his control.

The water-based theme is greatly significant to Murphy's story, as not only was his son raped and then drowned in a lake, but the prison hits on Napier and Coleridge had taken place in the shower room. Thus, Murphy's association with water is both a combination of trauma and tragedy. Murphy is frequently forced to run from the impervious red Void that is always in close pursuit whenever he's stuck in this reality.

Though it isn't delved into much depth, it's implied that Murphy does/did have some religious beliefs. Given that he's mentioned speaking with a Chaplain while in prison and he was raised around nuns, he was most likely raised of the Catholic persuasion. Though how much of those beliefs he's held onto after all that he's been through is debatable. If nothing else, he does believe in Hell. Because Murphy sees himself to be his own worst enemy, his Otherworld is a nightmarish place full of spikes, chase sequences, machinery, and confinement that resembles a very prison-like space of self-punishment. To him, it's a Hell that he deserves, and a place where he may have just been headed towards all along.