Each monster has a set amount of HP, or hit points, equal to the amount of damage that must be dealt to kill it. Monsters of the same zone will always have the same amount of HP, and with each zone, the number of HP all monsters have rises.
The formula for monster HP at any zone can be found here.
As the monsters get more HP, their Gold drop will also increase to keep up with the increased cost of the heroes that need to kill the monsters.
The formula for monster Gold drop at any zone can be found here.
To advance in zones and progress in the game, you have to kill a certain amount of monsters each zone. This number depends on the following factors:
The level (every 500 zones, the number of monsters required goes up by 0.1)
There are two types of zones: regular zones and boss zones. Regular zones require you to kill 10 (plus 0.1 for each 500 zones) monsters in order to unlock the next zone; boss zones only one. To make up for that, monsters on boss zones have more HP than a monster on a regular zone would have, and they have to be defeated within 30 seconds.
Kumawakamaru lowers the number of monsters required to finish a regular zone, up to a -8 reduction.
There are 11 different zone types, each containing a different set of regular monsters. Every zone type lasts 10 levels before it switches to the next one, with the exception of the first 10 zones, which contain 2 zone types instead of 1. Below is a list of which monsters appear on which type of zone.
Angry Potato It's a big potato and it's out to get you!
Catra Does she have any eyes?
Catra turns into this when she 'dies'.
Gerbeel It's a giant gerbil bee hybrid abomination!
Gerbeelpillar Another gerbil-bug monstrosity!
Gloop A more typical slime.
Finky Eek! A giant rat!
Flower Bloop Another slime! And it's a..girl?
Impossumble A rat this big seems impossumble!
Loggernaut Even the trees are after you! Or the stumps at least.
Mushroom Bloop It's a 'slime', made entirely out of dirt!
Sasquish Even Sasquish is after you! At least he's happy. And armless.
Spit Flower A giant flower, and it looks capable of shooting seeds!
Tyrantula A giant spider?! You're going to need a bigger boot.
Mamma Ferny Fat Bat A leaf covered bat, probably the biggest around.
Big Angry Potato Told you she was out to get you.
Caperticus A walking cactus is out to prick you!
Catra Catra is back with sharper claws!
Dirge Beetle A giant bug with a skull as a shell!
Finky Finky? More like Stinky!
Flowering Caperticus It's got a flower too!
Grablin Goboolin has a friend, and he's got some nabby hands!
Goboolin Positively frightening!
Sand Ball A living ball of sand, Doesn't seem very threatening...
Sand Bloop There are sand slimes too!
Big Dirge Beetle Now where did he find a skull that big?
Emperor Skorpinch A giant scorpion! Better keep your distance.
Dirge Beetle More bugs with skulls.
Finky Is he in every floor?
Flowering Caperticus Even dead, the flower smells quite nice.
Goboolin Goboolin is back, still sticking his tongue out.
Grablin Grablin is back, and upped his snatching game!
Sand Ball You don't even notice this creature as you kick through it.
Sand Bloop You take the time to appreciate the flower on the bloop.
Scorpinch Wasn't this just a boss? No, its eye is different.
Tyrantula Let's hope you increased your squashing game.
King Caperticus I think this might be the record for biggest cactus.
Trolgre The goblins told the trolgres that you were being mean!
Sea Snail I wonder what's in the shell..
Devilled Crab A little crab comes out from the sea just to grab your leg! What nerve!
Turtloid Warlock He thinks he's so mysterious behind that cloth..
That'll teach him!
Finky You begin to doubt you'll escape this rat.
Ratty Looks like Finky has a tougher friend.
Mouseketeer Even the mice are joining the fight, with swords!
Mousekewich Seems like the mice have a grip on magic.
Flamingogo A flamingo in leg scrunchies? I came to slay monsters not clowns!
Mudball A little ball of mud tries to dirty your shoe.
Turtloid Another turtle is up in arms with your conquering!
King Devilled Crab King crab? More like king feast!
Octotentacle A ship sinking tentacle for sure.
Gerbeel The Gerbeel is back in all his glory.
Sand Bloop I thought this was Mudlands not Sandlands.
Scorpinch More scorpions to watch out for.
Goboolin Jumping out from behind the trees.. don't scare me like that!
Grablin Hey! That's my Gold! I killed a lot of things for that!
Mousekewich His stick better be more magical this time.
Mouseketeer I wonder where he got that tomato...
Flowering Caperticus This place seems too wet for a cactus.
King Loggernaut Who ever thought a leaf was a good weapon?
Mud Golem A beast made from the mud under your feet.
Mousekewich You're making a tidy profit from that gold ring on his hat.
Mouseketeer Who tied the bows on these guy's tails anyway?
Goboolin They have followed you for miles just to get chopped down easily.
Grablin I bet the person who names these creatures wasn't very creative.
Flowering Caperticus I don't think Darwin ever envisioned cacti with feet.
Sand Bloop Another day another bloop.
Gerbeel How these creatures manage to fly is beyond me.
Scorpinch I wonder what he's looking at over there.
Mousity Is that an adult ratty?
Oversized Ratty Your heroes don't really want to eat rat meat for a week.
Big Feets The truer, dirtier, Sasquatch
Trolgre Looks like the Trolgre has been downgraded.
Mushrimp It's a mushroom. Of DOOM!!!
Foomgus The stench this thing puts off is almost unbearable.
Bluzebleeb It looks mutated. Creepy.
Goboolin I don't think these guys can put their tongue in their mouth.
Grablin Which creature's bones do these guys use as clubs?
d'Orcling Another addition to the goblin family. He's just a d'orc though.
Fat Bat Hah, he can't even fly!
Mudball Wasn't scary before and still isn't.
Sage Shroom At least these guys seem more magically inclined than the mice.
Mage Shroom A mushroom-jellyfish-mage? I've seen it all.
Floatsie Never mind, I haven't seen it all.
Superfat Fat Bat Like a Fat Bat, only fatter!
Dearth Bat I feel like this is a reference to something...
Stone Fields Monsters
Stoney Bloop Another bloop for your collection.
Zombie Bloop I didn't even know slimes could become zombies! Or have brains!
Big Feets This guy needs to get a trim. From your sword!
Mouseketeer What a fitting arena he's standing in.
Dearth Bat Wasn't he JUST a boss?
Sealed Elemental Sealed away for safety. That's why you must break it!
Cassoweary A bird with a stone tablet as a hat?
Stankape You don't even want to know what's on his face.
Sage Shroom I wonder where these thing's mouths are.
Mousity I think its just ratty's friend or maybe its still an adult ratty.
Stone Fields Bosses
King Stoney Bloop Mega Bloop!
Golem He looks like he's been waiting a century for someone to pass.
Snowlouse Referi must have fought this creature before.
Snow Ghost Looks more like a Banshee to me.
Snowball Aren't we supposed to throw these?
Ice Bat And now the Ice is alive too. Great.
Ghostly Fat Bat You can barely see him!
Snowkin A sword of ice seems like a pretty good weapon.
Snowmagus Look out he's got a broom!
Polaburrr You don't want to mess with him.
Snow Bloop Surely he shouldn't be able to move when he's this cold.
Snowdier He hits like a cannon!
Yeti He's the king of the mountain!
Apparition You better have some earmuffs with you.
Astral Rift Monsters
Box Monster It's just a box?
Ghostly Fat Bat It's easier to see him against the background of space!
Prime Slime He sees what you did here. And there. And over there.
Mud Golem Now how did a mass of dirt get into space?
Eye Slime My, what a big eye you have there..
Sage Shroom He seems almost at home here.
Sealed Elemental Wonder where all these elementals go when we free them.
Ghostly Bloop A new addition to the bloop and ghost family.
Astral Rift Bosses
Big Sage Shroom Maybe he is why the smaller ones like it here.
Arcane Guardian A being of such great power that it can lag the game itself! (Click on him really fast.)
Sage Shroom I hope the grossness of this place gets to him as much as it does me..
Mud Golem This much blood can't be good for anything. Plant or not.
Eye Slime He must have a really good barrier to not turn red here.
Box Monster Somehow not a wet pile of cardboard.
Ghostly Bloop I don't think ghosts care what the floor feels like.
Prime Slime He's seen some crazy stuff.
Ghostly Fat Bat Even the ghosts are too fat to fly? That's a little sad.
Sealed Elemental Wonder how bad it feels to be sealed in a crystal forever.
Fat Floatsie King of the 'Stare into Your Soul Club' group.
Putrefaction Ooze I REALLY don't wanna be that guy in there.
Treasure Chest can be found on any non-boss zone. They give 10× the zones's base Gold when killed and that number can be increased further by Mimzee. The base spawn rate of Treasure Chest is 1%, but it can be raised with Dora.
A Relic Ooze may replace any non-boss monster that would have spawned between zone 99 and 2/3rd of your highest zone, once per Ascension and only after your first Ascension. They drop Relic, equip-able items that provide various bonuses, similar to Ancients.
A Relic Ooze
Primal and Centurion Bosses
Starting at level 100, every boss zone has a chance to contain a Primal Boss instead. Primal Bosses are just like regular bosses, except for the fact that they give you Hero Souls, as well as the regular amount of Gold. Just like Gold, Hero Souls scales depending on your zone. Only the first boss on a zone can ever be primal. Once it has been defeated, the next boss that spawns will be a regular one, and will not award Hero Souls again. However, failing to beat a primal boss and returning to the zone will mean that the boss is primal.
In addition, every 100 zones up to zone 1000 a Centurion Boss can be found. The game will treat them as if the zone had a Primal Boss, the only difference being that Centurion Bosses have a 100% spawn rate, whereas Primal Bosses only spawn 25% of the time.
The 10 Centurion Bosses are unique monsters that cannot be found on any other zone. When you defeat these bosses for the first time, you will get an achievement.
Omeet He's just a widdle hamster!
The Green One Thought to be the incarnation of nature itself, he is a force to be reckoned with.
Woodchip, The Rodent Could use with a diet.
Queen of Bloops No mother is happy when her children get killed. A lot.
Doppler, the Robot He sees all that is in the heavens. Some say he can see back before it all began.
Rashon, the Duke Bow down to your lord,RASHON.
The Dark Wizard No one knows why he is here, but he is in your way!
Tako, Head of the Octopi So this is where all of those tentacles were coming from..
Tako Returns He was a robot this entire time!? Or maybe he wanted a rematch..
Lagomorph of Caerbannog It's just a BUNNY! Here I got 'em. *whole head gets bitten off*
When reaching a boss level, there is a very low chance that instead of the normal boss for that level, it will spawn this "Kappa" boss instead. It is a Turtloid Warlock enemy, but with the twitch.tv "Kappa" emote on its face.
Ramicorn A little far from the grazing grounds aren't you?
These boss monsters only appear in Clan raids, and as such, being in a Clan is required for fighting them. They don't take damage from your DPS, instead you have to click them or use Auto Clickers, but with damage that only increases when you get primal Hero Souls. Killing them rewards you with Hero Souls.
Defeat Omeet (Zone 100) How can one hamster be so hated throughout the land?
+1 Hero Soul
Bounty: The Green One
Defeat the Green One (Zone 200)
Bounty: Woodchip, the Rodent
Defeat Woodchip, the Rodent (Zone 300) He used to be a Kung Fu master, but he really let himself go. Probably one too many pizzas.
Bounty: Queen of Bloops
Defeat the Queen of Bloops (Zone 400) She's very angry that you've been smashing her babies.
Bounty: Doppler, the Robot
Defeat Doppler, the Robot (Zone 500)
Bounty: Rashon, the Duke
Defeat Rashon, the Duke (Zone 600) Once an Ancient, Duke Rashon became mortal when a group of 5 adventurers stole his Aegis. Good thing, too, because he's standing in your way of getting this achievement.
Bounty: The Dark Wizard
Defeat the Dark Wizard (Zone 700) A legendary boss from another realm, he's here on a secret mission.
Bounty: Tako, Head of the Octopi
Defeat Tako, Head of the Octopi (Zone 800) He'll probably come back if you kill him.
Bounty: Tako Returns
Defeat Tako, again (Zone 900) This isn't even his final form.