Cossack created them, but Wily had done some work on some of them, particularly the combat orientated ones. Wily just has this knack of taking robots and making them his. Which is why the majority of the established DWN list is more like a list of robots people used to take over the world. The R4 group are both "DWN" and "DCN".
You can see the same things with the R3 robots being dominantly listed as Wily Numbers (Right and Wily Numbers, except ShadowMan), the R6 robots being listed as DWN and MXN (Mr. X Number).
The odd thing about that listing is that regardless of DRN, DWN, DCN and MXN, they all follow on each other. Whereas such things as the King Numbers however follow a completely different listing, just as the Space Rulers do. It's further complicated by TimeMan/OilMan, which are DRN-009/010, even though DWN-009/010 is MetalMan/AirMan.
Finally, I believe it was R4 in which the credits and the sourcebooks don't agree at all about the numbering of these robots.
All in all, I personally feel the DWN listing is very much in need of a gigantic overhaul, there really isn't that much consistency.