Cocoons are a unique "enemy" that can't retaliate, but have a lot of health and eventually spawn an enemy.
Cocoons just stay in one place; they don't move or attack in any way (although a cocoon can be pushed by Albert West and mutants, but it will bounce back to its original position). Occasionally, an enemy will spawn from the cocoon, which can be anything from a Normal Mutant to a Pre-Martian. When an enemy spawns from a cocoon, it is destroyed without dropping any items. Cocoons can be found in packs of up to four. In addition, cocoons come in four different sizes, each with varying health. No other difference has been observed in cocoons' sizes, but the size may be indicative of what will hatch.
Occasionally, an unalerted mutant can be observed "transforming" into a cocoon, and can create up to four at once. The mutant itself disappears.
Cocoons have a lot of health, so it is best to attack them with a shotgun up close or with a powerful melee weapon. However, since they don't retaliate, almost anything can work. An SMG may be preferable for its low heat usage, even though it will take longer than a shotgun, or a pistol can be used for its compromise between power and energy usage. Although they can also be left alone since they pose no immediate threat, it is advised to take out cocoons whenever they are spotted, so that enemies can't spawn from them, especially if you have to return to the area later.