Sadly, this couldn't be further from the truth. Nezumi is simply the Japanese word for "rat or mouse", and technically "Ratman" should be "Nezumi-jin". There was at least one giant rat who could be interpreted as a Rat-man in Japanese folklore, the Yokai Tesso, who I wrote about way back in my Prehistoria Rodentia, but in general the Nezumi have no basis in Japanese mythology. This does not, in my opinion, make them any less awesome...
Alderac Entertainment Group is a publisher of RPGs, CCGs, and board games founded in 1993 by Jolly Blackburn, nowadays best known as the creator of the Knights of the Dinner Table comic strip. Legend of the 5 Rings began life as a CCG in '95, then a roleplaying game in 1997.
GCT offers two Nezumi figures in their Cult of Yurei range:
Rat Nangun Warrior Monk...
An Italian company that's come out of the Kickstarter Era, Greebo mainly specializes in themed Bloodbowl teams (Bloodbowl Ratmen in general will be the focus of an upcoming post), including The Hand of Death, a range of Japanese-themed Ratmen that is, overall, absolutely stunning.
I regrettably missed the Hand of Death Kickstarter, but recently put in a large order with Greebo, and I have to say these are some of the best sculpted figures I've ever seen. Very high quality, no flash or overt moldlines, and impeccably detailed. For anyone looking to fill out their Eshin ranks, I simply cannot recommend them highly enough.
So without further ado...