Goodman Games

How has the lunar rain affected the normal monsters of the world? Are those niches now filled with moon creatures or what?

There's still plenty of room for normal creatures on DragonMech. Just because we have a lunar equivalent of zombies doesn't mean there are no more zombies. In fact, you need non-lunar creatures if you want something without a tie to madness incarnate. Many treants are now the mad forestrati. There are oil-slick versions of water nymphs. Etc. etc. See Mech Manual for more monsters (many of which aren't lunar). Also, the Rumors & Lies pdf has a template for earthly creatures that have been warped into lunar creatures. In general, many monsters roost in city-mechs if possible - especially in the legs of Sharlorn the Necropolis.