Cobra Toxo-Zombie, Hostile Environment Trooper, was released as part of the 2014 Zombie Initiative G.I.Joe Convention Exclusive. Toxo-Zombies (with an "S") only refers to more than 1 Toxo-Zombie. Each boxed set came with 3 Cobra Toxo-Zombies and 3 Cobra Toxo-Zombie filecards. His helmet is part of his headscuplt and is not removable.
The only collector's issue in obtaining a Cobra Toxo-Zombie v1 is that the Convention Set was made in limited quantities.