This thing is a clot.
It wants to kill you.
Shoot it in the head jaw. (the hitboxes are buggered)
Pretty much every class can deal with clots, although it's more ammo-efficient if you leave the killing of the "trash" to a medic, commando or firebug.

It doesn't really pose a threat unless it is part of a large group, or behind you as he can grab you and prevent you from moving. (Think if it as a small kid who wants to be cool grabbing another kid and letting the bigger bullies kick the living snot through him)

Clots appear on wave 1, and every other wave including wave 11 if The Patriach reaches his first healing threshold.

The Bloat is huge, you can't miss him.
The Bloat also sponges A LOT of damage, so a headshot is the only way to kill him unless you want to waste at least a mag or two, just pop him in the head and let him bleed out.
His vomit attack is surprisingly strong, and it obscures vision, so it is best to keep your distance when tackling him, and bloats rarely spawn in larger numbers than two.

He rolls in on wave 1-10, not joining The Patriach.

This is the Gorefast.
He lives up to his name.
He works in tandem with ZEDs such as the clot and crawler, with him being the damage.
Simply shoot him in the head to eradicate him (His hitbox is fine), but he may wander around aimlessly when decapitated, it may be wise to put the extra few bullets into him and kill him dead, as i've taken quite the chunk of damage after thinking he's not a threat.
Once again it may be wise to leave them to the trash clearers if you're busy dealing with a bigger mob, although if you're playing sharpshooter, you should probably just pop him with your 9mm and eliminate his ability to run.

The Gorefast appears on wave 1-10. He does not accompany The Patriach in the final battle.

The crawler is small and black.
If the map is dark (most maps are), he'll be a lot harder to see than a stalker. Trust me.
It is REALLY weak, 1-3 shots from pretty much any weapon should kill it.
Although it'll nip at your ankles, he shouldn't pose a large threat unless undetected, he can be a pain in the arse for zerkers, so just put a bullet or two into the crawler and everything will be fine. Probably.

Crawlers appear on wave 2-11, fighting alongside The Patriach if he reaches his second healing threshold.

Say your prayers, she's a ♥♥♥♥♥.
Her health is not brilliant, but she can sponge a bullet or 10. Not to mention that her head is a lot more durable than the other ZEDs.
She will scream.
She will destroy grenades.
She will ignore armor.
She is a PRIORITY target.
If left on the field for too long, she can impune demolitions entirely, and stop medics from healing as she also destroys healing darts.
Sharpshooters should have no problem with headshots, as the extra durability of her head doesn't stand a chance against the sharpshooter's bonuses, other classes should just hose her down as her body only has 150 more hit-points in the harder difficulties and her body is an easier target and this leads to less bullet loss.

She first appears on wave 3-10, not joining The Patriach.

The Stalker poses little to no threat at all, even though it can outpace most classes that do not have a speed boost, when it attacks she stands completely still and swipes, so if you're still moving, you wont be hit and the Stalker becomes nothing more than a meat shield.
I may have forgot to mention that the Stalker's forte is turning invisible.
It can remain undetected for a while, although if you see a shadow pacing towards you at a pretty decent speed, you should probably shoot it.
The health of the Stalker is pretty poor, and headshots are quite difficult due to it being invisible for 95% of it's lifetime, just shoot it. The commando has the ability to see cloaked stalkers, therefor making him the go-to man for stalker elimination.

The Stalker joins the fight on wave 2-10, Also joining The Patriach in his assault on the players. It joins the fight during The Patriach's third and final heal, therefor being the last ZED to join him.

The Husk is a tricky little bugger.
He's pretty big and stands out in a crowd due to his metal frame and huge fireball launcher. Yeah, fireball launcher.
He, along with the siren, are the priority targets of the early waves as he can chip away at the team from a distance.
Your best bet is to shoot him in the head, only the sharpshooter is capable of a single shot headshot, and that's only with the rifles. Although firebugs will have a difficult time as Husks have a 75% resistance to fire, this balances with the firebug's own resistance. Try to help out the firebug as he will find it incredibly difficult to kill one on his own, and it takes his focus off of the trash.

The husk appears on wave 2-10. He does not aid The Patriach.

Is it a bird? Is it a plane?
Nope, that's a Scrake.
If you have difficulties locating the Scrake, he's the really, REALLY deranged looking one with a chainsaw for a hand and doctor's scrubs.
Funny thing about him is that although he looks menacing, he's really pretty useless to the ZED team until you trigger his special mechanic. His rage.
His rage is based primarily on its health, the difficulty of the game and a random variable; the harder the difficulty, the more likely they are to charge the players.
You should leave him to the sharpshooters, zerkers or supports, a sharpshooter can take him out with a crossbow headshot on most difficulties, or a few LAR shots. The zerker can stun the Scrake in a menagerie of ways (you'd be better searching up a video or guide on this, i dont play zerker that much), and the support can unload on him with his numerous shotguns.
The commando can also give insight as to the Scrake's health, and probably guess when he's about to rage.

The Scrake stumbles into the game on waves 5-10, choosing not to join The Patriach.

WHAT IN THE-
I SAY WHAT IN THE ♥♥♥♥ IS THAT

That is a Fleshpound.
The Fleshpound gives and takes a LOT of damage, and to top it off, chooses to charge at you whenever he feels like it. Well, when his predetermined timer chooses to anyway.
As soon as the Fleshpound sees someone, a theoretical timer is started, 10 seconds + a random number between 1 and 5, so if he is coming after you for longer than 11 seconds, he's essentially like a bomb.
He also rages if he takes more than *320* damage in 3 seconds.
The rage timer can be halted by him swinging at /or/ hitting you, or if you break line of sight with him.
The rage can be stopped in 2 ways, by getting hit by him, or by putting a welded door inbetween you and him.
His rage is signaled by him roaring and his light turning from yellow to red.
It is a TEAM effort to kill this bastard, with everyone needing to chip in.
(or just 1 demo) - nazi sauce
He is PRIORITY 1 due to his ability to wipe a whole team.
There are a LOT of specialty ways to kill a Fleshpound by yourself (again, you'd be better looking it up)

He only charges into the fight on wave 7-10, also choosing to abandon his creator in the final fight.