Gu'chi Swarmling
Drops from Silkworm Cocoon. Swarmlings of Gu'chi spin an ancient, and very powerful, form of silk that has not been seen in Pandaria in ages. Critter type. Buying this companion (or any others from the Isle) does NOT count to "Going To Need A Bigger Bag" You have to loot them.
The Gu'chi Swarmling is a critter-family battle pet obtained as a drop from Gu'chi the Swarmbringer, a rare spawn in Pandaria.
Guide awaiting review
Locate Gu'chi the Swarmbringer
Travel to the spawn location at 41.1, 81.2 and wait for Gu'chi the Swarmbringer to appear. The rare spawns on a timer ranging from 31 to 59 minutes between kills.
Kill and Loot the Rare
Defeat Gu'chi the Swarmbringer at 41.1, 81.2 and loot the 
Silkworm Cocoon directly from the body. Wowhead reports a 3% drop rate, approximately 1 in 34 kills, so multiple attempts should be expected.
Loot to Count for Achievement
You must physically loot the Silkworm Cocoon from the corpse for it to count toward the Going To Need A Bigger Bag achievement — purchasing the pet through other means does not qualify.
Upgrade Pet Quality If Needed
The Gu'chi Swarmling can drop at Uncommon quality. If yours is not Rare quality, use a Critter Battle-stone to upgrade it to Rare.
Tips
On Tuesdays, significantly more players tend to be present at the spawn location, which reduces the chance of a solo kill.
