from GTA wiki -
The Ratman is a common myth in Grand Theft Auto IV. It's existence is currently unproven, but many of GTA IV's fans have argued over whether it does exist or not. Most pictures that appear on the internet are real in-game clips, but because there is currently no active mod for the Ratman, they are most likely just photoshopped images. Some pictures are also just the result of a common mistake, such as the mistaking of homeless people in the subway for the Ratman.
At a convention, a GTA CEO stated that "There currently is no Ratman, but it is possible that in games like these with many graphics, something could erupt and take on a role of it's own".
have you found ratman, i know i have?, post you videos and pics here!
EDIT: cant seem to post photos from my pc at moment
EDIT 2: Pics in my gallery