Yet again, this article on Liberty Tree.com can uncover the origins of Ratman.
Zaibatsu is likely behind this mystery. However, I don't know much about the mythical creature, Ratman. But I can explain the strange rat infestation in the subways; as you read the article on my right, it clearly tells you that Zaibatsu was preparing a medicine to ensure psychotic behavior in rats, strange, isn't it?