The Cocoon is the transitional Zerg unit Mutalisks turn into during the morphing process to Guardians or Devourers. This morphing process takes 40 seconds at normal game speed, effectively the life span of the Cocoon. Although the Cocoon has 200 hit points of health, it does not have ten armor like the Zerg Egg. This makes it much more vulnerable to attack by opponents, in comparison. It is noted, however, that Mutalisks frequently morph into Cocoons near the opponent's bases, unlike the Egg, which morphs at home bases and usually having easy access to Zerg troops or defensive structures (e.g. Sunken Colonies). Cocoon morphing often take place in an isolated position that is slightly out of the opponent's vision, as Cocoons are static with no attacking ability during the morphing process. It is common to see Cocoons morphing in a body of water, over a cliff, or other relatively inaccessible areas to avoid opponent attack.
When a unit is about to hatch from the Cocoon, it starts the hatching animation. During this animation the new unit maintains the HP percentage of the Cocoon, but is restored to 100% health after the hatching animation completes. It is worth noting that Cocoons benefit from Zerg Carapace upgrades, instead of Zerg Flyer Carapace.
Because of the Cocoon deal any damage and is static/frozen during its life span, it is not an attack unit in any match-up, but rather a burden to the Zerg play to protect and vulnerability that is exploited. If visible, the opponent will usually attempt to attack the Cocoon before it completes morphing.
Cocoons are rarely seen during the ZvZ match-up as games rarely progress to the Hive and Greater Spire stage. If seen, they are generally Devourers. Cocoons are vulnerable to any air attack unit, which is usually the Mutalisk or Scourge in this match-up
Cocoons are sometimes seen as Guardians are used as a harassment tool in the ZvT match-up. Because it is a late stage unit, the Terran usually has Science Vessels constructed with the Irradiate upgrade, which deals 250 damage, thus destroying the affected Cocoon and any neighboring air units. Marines, Goliaths, Wraiths, and Valkyries are also commonly seen in attacking the Cocoon.
Cocoons are sometimes seen as Guardians are used as a harassment tool in the ZvP match-up. Furthermore, if the Protoss player elects to build substantial air units such as Carriers, Corsairs, and/or Scouts, the Zerg player may opt for Devourers, as well. The Protoss player will commonly attempt to Psi Storm the Cocoon before it completes morphing, but two Psi Storms are necessary to completely kill the Cocoon.