If I saw something like this wandering through my house, I would mess my
pants, run out screaming, and eventually end out in some program to set
my mind back into its nice little safety zone of flowers, butterflies,
and the occasional beer.  This video, posted by pdszsz
on Nov 25, 2009 gives me the willies.

Though extremely well rendered, this is fake.  The creature is a Venom Headcrab from the popular video game Half-Life.  This is truly a really well-made special effect.  Here is a breakdown of how it was composited:

If anyone out there designs unique creatures, drop us a line!  We may just post your videos here!  And we all know that there's nothin' better than free publicity...

For more crazy animal videos click here!