Agron

Frequently misspelled "Argon" despite being a solid, this is an extremely powerful yet slow beast. It is unclear exactly how Agrons came into existence. What is known is that they are much tougher and capable of surviving attacks that would disintegrate their zombie brethren. The Agron's large blades are capable of ripping through even the strongest armor, often causing fatal open wounds within the victim. Only tanks should attempt to face one of these head on, and then, only properly equipped with Reactive Armor.

Detector
An Agron's eyes are but one of the many senses that it has. It is capable of ascertaining the position of any enemy around it, and relay this to other members of the horde. Due to this ability, an Agron can detect invisible units like a cloaked Mobility Recon and Subtlety Marksmen. They can also detect any burrowed enemies.

Parasitic Incubation
On higher difficulties and in Nightmare Mode, Agrons will spawn 3 parasites upon death. If they die while burrowed, Parasites may not spawn.

Burrow
Agrons will attempt to burrow when near death. Once burrowed, they will heal and reemerge at a later point. Some form of detection, like a thermal sight is needed to reveal burrowed Agrons.

See up Ledges
Due to the Agron's acute senses and massive size, an Agron is capable of pinpointing targets that normally are hidden on higher ground. This makes an Agron dangerous in hordes with other ranged zombies such as Infested Marines. The Agron can easily provide vision of Marines on high ground.