Greetings! Everyone else seems to have a "Realistic *," so I figure I should have one, too!
Here is my try at a realistic explanation of each alien class:
Dretch
Not much to say about the dretch. Except it's a magic, walking, steak. It can probably talk, too. And grilled dretches taste mightily good with grangar sauce.
Basilisk
A hideous experiment gone wrong: a cross between a ninja, a spider, and duct tape. We have reason to believe that this experiment came out of the secret labs of the French government, so blame them for it (as you should everything that goes wrong).
(http://barkingfrogs.net/images/79485_1_ninja.jpg)
Leaked Basilisk Concept Art
Advanced Basilisk
Like the Basilisk, but carries around RAID cans (the Frenchies must have engineered it to be resistant to bug spray! Darn them!)
Marauder
The marauder is covered in tiny bouncy balls that allows it to bounce off pretty much anything with the greatest of ease.
(http://www.minniemites.co.uk/images/ballpit/ballpit2.JPG)
Like this, but tiny. And everywhere.
Most include a small music player that only has the sounds of baby polar bears dying, and they play it when hurt. While this is supposed to appeal to people's sense of humanity and cause humans to stop attacking, it has the opposite effect: PETA became aware that the aliens tortured baby polar bears in order to create this sound, and put out a bounty on the killing of marauders.
Advanced Marauder
Like the Marauder, but also features microscopic Tesla generators between their balls. Strangely, these Tesla generators have a small French flag on them. Coincidence? I think NOT! >:(
Dragoon
Unremarkable, except for it's powerful hind legs that allow it to pounce. Given how powerful it's legs are, it's essentially the Arnold Schwarzenegger of aliens.
(http://forumpolitics.com/pics/arnold_schwarzenegger.jpg)
Now imagine him jumping about and eating your teammates.
Advanced Dragoon
Like the dragoon, but shoots little barbs out of it's mouth. Could these be little French barbs!?
Tyrant
Just a big, huge, tank of a creature. Could be a failed attempt to create a giant centaur.
Granger
Yes, these things are incredibly cute, but don't let it fool you. The granger is the alien equivalent of the Furby -- don't turn your back on these devious creatures.
(http://www.trem-servers.com/images/tremx_models/adv_granger-r.png)
This granger was pictured immediately after it slaughtered 42 innocents.
Advanced Granger
This one has glue on it's feet. And shoots barbs. It's a cross between the regular granger, the dretch, and the advanced dragoon -- very frightening.