Alien Swarm Parasite

The Alien Swarm Parasite is an unlockable headwear item for all Classes. It is obtained by earning the Hat Trick achievement in the game Alien Swarm by completing 2 co-op missions (any difficulty setting) online.

Trivia

Hat Trick Achievement Icon

• This is the second hat that appears to be a still-living creature, the other being the Triboniophorus Tyrannus.
• The Parasite's tail has jiggle-bones making it the second promotional hat with jiggle-bones, the other being Max's Severed Head.
• This hat is the first and only item to be unlocked via the achievements of another game.
• This hat was speculated days before the game was even released due to the Heavy appearing on the icon for the Hat Trick achievement.
• This is the second hat that allows the Soldier's bald head to be shown, the other being the Cheater's Lament.
• Here is bug, that the hat does not weared properly by classes. It should take all. See here.