The egg created by the Infestor’s Spawn Infested Terran ability currently uses an outdated version of the model.
In Heart of the Swarm, a new egg model was created for the Infestor’s Spawn Infested Terran ability. The old cocoon resembled a generic green egg, while the updated cocoon had a fleshy appearance and appendages that resembled Infested Terrans’. However, the updated model was accidentally only applied to competitive mode; due to a bug, the campaign dependencies mistakenly overwrite the versus model with the outdated WoL model. (This is because the Liberty campaign dependencies include an outdated duplicate of the data with parent “MissileFX”, which effectively replaces and overwrites the versus data instead of building off of it.) This issue applies to every mode WoL-onward that uses campaign dependencies, including coop.
The updated model created in Heart of the Swarm is called “InfestedMarineEggMissileEx1.m3”. (Ever since the ability was removed from versus, this model has been completely unused.) The outdated model currently used in all campaign dependencies is called “InfestedTerranSpawnMissile.m3”.
(Curiously, Infestors on Malwarfare aren’t affected by this issue, as they use a unique skin of the HotS model (“Hologram_Skin_InfestedMarineEggMissile.m3”). So only on Malwarfare does the cocoon resemble what it’s supposed to. Stetmann’s Mecha Infestors use a variation of the WoL model (“InfestedEgg_Stetmann_COOP.m3”), but this is appropriate as their ability summons a different unit type that doesn’t visually match the HotS cocoon.)