We all know what these suckers are. They were brought here on accident, and they are basically wild animals that kill everything.

We all know what these actually are as well. These were brought on purpose as Bio Weapons, but backfired, and are now threats to everyone.

Those are Flares. They are literally beings made of Crazy Space Dust. They warp things in and out of universes constantly, and use whatever they warp as weapons.

These are called Gargantuas. They are huge creatures (height of a Xenomorph Praetorian.) and are fully immune to all types of bullets, their eye is also immune, but explosives, radiation, and electricity is effective against it. They have flamethrower hands and an orb that causes internal bleeding that they create from their feet.

The Combine have also breached this universe, and are setting up shop slowly. They also use everything to their advantage and remain unknown. 

Might add more later.