ranked from least to most severe, threat levels per specimen go like:
trivial < meh < annoying < decent < bastard < yikes

- the axe exists, use it (busting cobwebs, self-defense, etc)
- always be running, you want to get as far as you can as fast as you can
- memorize room layouts, don't get caught in the wrong place
- be smart about your stamina, some of these boys will completely merk you if you end up cornered or run out of breath - one must get used to not constantly holding down sprint in order to keep moving on pace
- be smart about monster behavior, lead the ones that phase through walls to your position first before they head you off
- sh*t gets real after room 900
- real gamers use the sword instead of the axe
- don't break any giant vials with brains in them
- don't walk down any endless hallways
- don't look directly into the fan
- get good idiot

threat level meh
kills in 3 with a long-ish cooldown, sometimes he floats over pits, sometimes he randomly appears without introduction
smack him with the axe and it'll stun him for a bit
really only ever poses a threat in conjunction with any other specimen that moves speedily
moves at about walking speed, maybe keep that axe on a little bit of standby when you're wading through his nastee fishtank water
protip: smack him while he's floating over a pit and he'll go away for the room

threat level meh
drops from the ceiling, progressively gets faster the longer it goes in one direction, kills in 4 with quick cooldown
smack it with the axe and it'll run the other way for a bit
keep your axe on standby and watch the ceiling, keep looking over your shoulder if you're being chased through a particularly lengthy or spacious room

threat level annoying / threat level YIKES in 2nd form
floats through walls at walking speed, three-shots you given the chance, unfazed by the axe
sometimes she goes ape mode and moves quite a bit faster, while obscuring your vision with encroaching static
keep moving and don't let her head you off. her second form is a serious run killer - coordination and concentration is key, try and get a good read of the room if possible before her static hard-resets your vision
protip: she moves slightly slower if you keep your eyes on her

threat level decent
messes with the room's textures, causing disorientation. makes up for her lack of speed with the fact that she can double-tap you given the chance,
immune to axes
her texture-warping can be extremely disorienting in conjunction with dimly-lit rooms, and while she's not to be taken lightly, she's easily outrun-able
better watch out better not cry shes faster than walking speed

threat level trivial
attacks whenever you take your eyes off him, three-shots you if you let him
smacking him with the axe incapacitates him for the entire room, and he spawns shortly after you enter - more of an annoyance than something to worry about

threat level meh
unstoppable wall of hurt and pain that instantly erases your existence upon contact
slow as hell
just keep moving forward and stick to the left and you should be fineeee
if things aren't fine just sorta move quick before annoying things happen
this thing is also a one-time encounter, which is kindof a shame in my opinion

i love karl jung's work :)

threat level annoying
as Cat pointed out in my absence, he's acquired an UPGRADE - while he's still slower than samara morgan, he'll fastly warp through any walls you put between him, which can lead to getting dead if you don't have health to spare
kills in 4 if you let him, keep moving and don't let him head you off, yadda yadda
if you REALLY don't want to peep into his funny cloak then make sure you have stamina to use the moment before he finishes warping
only worry about him if he's teaming up with another specimen really

I HATE WALLS

literally just make sure you pause

threat level decent
becomes outrunnable if you move too far away from him, changes back once he shanks you, immune to axes, kills in 3
also obscures vision with damage taken
becomes a run killer in tandem with a faster specimen, be smart about your regen and keep him within eyeshot
its finicky but possible to deal with his faster form, turn corners whenever you're able and keep an eye on your stamina - don't expect to not get bitten eventually though

threat level meh
floats through walls, kills in 3, makes doors turn invisible
smack him with the axe and he'll teleport back to the room's entrance
this dude's the other reason why it's handy to have rooms down to muscle memory, otherwise he's not that worrisome

threat level decent
moves slightly slower than running speed, can sprint to make up for lost ground, can three-shot you within 4 and a half seconds, blocks axes
keeeep moving, make sure you've gone around a corner before he charges at you and reaps you nasty self
moves a little slower when paired with Feejee lol

threat level annoying
moves whenever you're in water, loses you when you're on a crate, comes outta ducts sometimes to head you off, can't be axed, kills in 3
just keep moving and stand on a crate every so often
despite the fact that you move quite a bit slower in water, she can only be accompanied by Slimer, Babby or Ulcers (plus cloning), which takes away opportunities for deadlier dudes to end your run
protip: strafing away in open spaces while she's charging at you can result in a dodge, as it takes a second for her to turn herself in your direction
4132 is my favorite number

threat level bastard
only interested in grinding progress to a halt, shows up in low-res rooms in a random location and steadily makes a beeline toward you faster and faster to snack on your health, sometimes he does a Friday and shows up at doors, kills in 4 (in rapid succession if unlucky), can't be axed
also he makes you go through rooms with tight corners and long hallways ad nauseam
just try and skate around him, if you can't just tank it - should he kill you, type "open" in the keyboard the game gives you, immediately start booking it - your health will be locked at a sliver, and your room count will refuse to budge; the faster you get it over with the better

play Imscared! its a good time

threat level YIKES
blocks doors for a bit, then everything goes to hell and he starts slowly sipping away your will to live, sometimes he takes a beeg sip and momentarily slows you down. in this way he can kill you at any damn time if nobody else is chasing
also prevents regen so if you start getting chased you now have quite a bit more to worry about
sometimes he nags you for nearly the entire 100 rooms before a save
he's just bad news bears all around - keep adding to your room count if your continued existence is one of your priorities

threat level decent
floats through walls at above-walking speed, spawns ghost animals to block your path and gobble up your stamina, on top of screwing with vision
she's immune to axes, her animals aren't, kills in 3
coupled with any other legitimate specimen, she can evaporate your run before your very eyes-
keep moving, don't let her head you off, conserve as much stamina as possible

yiff in hell

threat level YIKES
unlike salesman and raggedy andy, she's immune to axes - you absolutely have to stare in her direction at all times once she shows up. she also messes with the room's lighting
on top of that, after five rooms she'll freeze the room count for a long period of time and start looping the past few rooms you've been through, drawing out your run and giving you plenty of opportunities to get your ass handed to you.
as you've probably gathered, she eats runs for breakfast when she has company with her, which she often does
i can say from experience that every single run i've had with with this wench in my specimen pool has ended in heartbreak. be cool, calm and collected
UPDATE - she's not terribly threatening in terms of damage now (kills in 7), but her chicanery aside from that is still very much lethal

threat level decent
moves hella fast (like HELLA fast) and three-shots you
can't be axed
keep moving and be resourceful with your stamina for GOD sake
your run is about to get way harder if he teams up with any other specimen that impedes movement
UPDATE: as i personally failed to notice, this man returns to the room's beginning on contact, which can be abused with invincibility frames if another member of the fright cabal is present

hehe haha HAHAA

threat level trivial
spawns somewhere in the room and takes up space, does little damage
axe him and he'll teleport somewhere else
only interested in blocking your way, nothin to worry about

threat level annoying
reskin of Benjamin except he doesn't spawn immediately, kills in 5 if you let him
keep your axe at the ready and he'll buzz off for the rest of the room once you smack him
just keep moving if you don't want to waste time really - he tends to swipe at you from angles rather than directly behind you
to this end, i've found that doing some Wiggly Strafing while running has led to less swipes :D

threat level meh
does leetle babby damage, axe him and he'll teleport back to where you came in
really the only ability of note he has is his way of messing with your vision and the walls, otherwise he's an actual joke
he can actually outrun your walking speed now, still doesnt change the fact that this man isn't exactly a threat
protip: if you REALLY don't like his attitude, keep axing him over and over and he'll go away permanently until the next time you see him
ALSO also, as Simpur has pointed out in his own guide that you should be reading instead, keeping Mad Cow present for the purpose of seeing through walls can be slightly handy!

threat level bastard
spawns in front of doors and floats your way, kills in FOUR FRIENDLY FACE FLAYINGS, can't be axed
if you arent running from anything just tank it or try and juke her, if you are then youre just straight up dead unless you're being chased by something weaksauce

i swear to god her and alex mahan have the exact same smile

threat level meh
following their start room, these kids will appear again without warning, sometimes in their pile form, sometimes not, sometimes bursting out of their pile form - axe them some questions
by themselves they're trivial and so is their damage, while being catch-22'd they can be a tad stressful
in the event of a barricade just lead away and or incapacitate the actual chaser before axing them out the way
otherwise killing all of them gets them to buzz off - you can theoretically waltz past them without smacking them if you're in the interest of time, just expect to get your ankles bitten a few times
ALSO, as Simpur pointed out in his own guide that you should genuinely read instead like don't waste your time, keeping one of these boys present can keep a specimen slot occupied where something worse could be instead!

i wish clicky was thrown in alongside clown friend, i love clicky

threat level low meh
spawns somewhere in the room at random, kills in four at a snail's pace, can be made catatonic if given enough head trauma
contrary to his speed, his windup time renders him not all that threatening even when out of breath, as he can be easily skirted around
his ability to harm depends on whether or not you're panicking or being ganked really
just keep strollin

yknow i like what he does and all, but tomar has some serious issues goin on man. like i don't know how chris and the others tolerate him sometimes, especially after he burnt down that orphanage with white phosphorus, like that sh*t's inexcusable

threat level annoying
her schtick is she DOESN'T like getting seen -- static indicates whether or not you're looking at her

her damage by itself isn't very scary, but she's still capable of quickly comboing your ass if you don't follow the rules of engagement, and she's especially obnoxious in tandem with decent-or-above specimens like Lisa The Painful
the trick here is to do your best VIRGIN JOG™ and keep your view at a downward angle without smacking into her, you wouldn't want to do that to your crush
in the event that you're ABOUT to smack into her, quickly stare at your feet - this has reduced the amount of instances where i've personally taken damage from her