You can get this in Single Player or Co-op. This achievement only counts for the player that killed the chubby so you will have to find another chubby if your friend wants the achievement too! (Playing with friends will increase your chances of finding Chubbies.)
Chubby enemies are a cute and fat version of normal enemies.
These are the types of chubbies;
*Skag - Three Horns, The Dust, Tundra Express, Wildlife Exploitation Preserve, Eridium Blight, Arid Nexus - Badlands, Arid Nexus - Boneyard.
*Spiderant - Frostburn Canyon, The Dust, Caustic Caverns.
*Varkid - Caustic Caverns, Tundra Express.
*Stalker - The Highlands, The Fridge, Wildlife Exploitation Preserve.
*Midget - Southern Shelf, Three Horns, Southpaw, Frostburn Canyon, The Dust, Bloodshot Stronghold, Bloodshot Ramparts, Tundra Express, The Fridge, Thousand Cuts, Lynchwood, Sawtooth Cauldron.
*Rakk - In the sky, Torgue DLC - Southern Race Way side quest "Gas Guzzlers".
The text will say "Chubby" in front of these names. Make sure you read it before killing with your gun. Chubby can be found all around Pandora, but there are a few spots they are known to show up.
Note when going for chubby varkids;
*YOU HAVE TO WAIT UNTILL THEY EVOLVE! Hatch from their pod.
*Put alittle fire damage on a small varkid for it to evolve into a pod let a varkid go into an adult pod and then a bad ass pod. It will either spawn as a Badass Varkid or a Chubby Varkid.
For a list on the known locations;
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Caustic Caverns Chubbies can spawn inside the Caustic Caverns. This is where I found mine! I found the Varkid chubby in the "Infested Warehouse" area. You might have to put a little fire damage on a small varkid for it to evolve into a pod let a varkid go into an adult pod and then a bad ass pod. It will either spawn as a Badass Varkid or a Chubby Varkid. I did this with my boyfriend in Co-Op 2 player. I later found 3 Chubby Varkids in the Casustic Caverns all in different areas when finding the Minecraft easter egg. All my friends in Co-op 3 player got this achievement one chubby each. We found Bunny Rockets that one looted.
Watch the video it will show you in the beginning of the level, this is where PowerPyx got his by one of them coming out of the dump truck. The Truck will shake a few times when they are about to come out sometimes it's a Badass Varkid so be careful! You can also get this in True Vault Hunter Mode I think it increases your chances since you are on a higher difficulty.
Thanks for PowerPyx for video Leave a Like:*
Sounthern Raceway Mister Torgue's Campaign of Carnage DLCSir Hammerlock gives you a side mission called "Gas Guzzlers" he asks you to go kill 10 Rakks in the Southern Raceway area. The quest marker will lead you right there just look in the sky and you will see one. (Increases your chance of finding one with friends playing the DLC)
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Tundra ExpressI would suggest traveling to the Farm but any area with varkids will work in the Tundra Express. You'll have to wait until the varkid pods evolve/hatch. (Kind of like a Pokemon) They will keep evolving just damage them a little bit with the Watermelon fires or use a fire gun that doesn't do a whole lot of damage. This will make them transform even faster. Be careful though because you could make them into badass Varkids or even the Mighty Vermivorous Invincible. But mostly need four people to do that for him to show up and beat him. If you don't see a chubby just rinse and repeat it again.
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Underground area's of Frostburn CanyonBlacktoe Cavern is where you'll find a Chubbie Spiderant it spawns in the Cavern after you pass some white snow like water, run past it up alittle cavern hill and a couple spiders will be here and the Chubbie can spawn here he's fat and yellow looking. But try to aim for his back you might get a better loot for a critical.
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The DustBy Ellie's Garage in the piles of Car Parts. Where the Spiderants that come up from the ground. Should be Brown and Yellow also fat.
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Slaughter DomeMidget Chubbie can spawn here it's very rare not sure where in the Slaughter Dome but maybe playing it a couple times since you have to beat 5 Rounds of it, he might appear.
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Three Horns ValleyYou can check in this spot by the pig motel. A Skag Chubbie might appear here. When you pick up one of the parts to restore the power for a side mission.
-Slightly to the south east of the bandit docks "Split Skull Bay", where the huge pump is located chubbies can spawn here.
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Wildlife PreservationThere is a Skag Chubby somewhere on this map when you are doing the story mission for Mordecai called "Wildlife Preservation" or just going back in free play of course. But it has to be done during the day! Better chance of finding one during the day in 4 player Co-op.