I believe that the spawn for flies is in the forest, the spawn for ogres is on the eastern coast past the ruined castle, and the mummies probably just wandered...last night I ran into 6 skeletons in the southern forest all looking out to sea....really scary!
As far as I know about the old server, the map only supports a certain number of monsters at a time, so if it passes that limit through something like summoning, no new monsters will spawn until the existing ones have been defeated to a number below the limit. Maybe there's a big pool of summoned monsters somewhere?
Or maybe the monsters' better AI are taking them to new frontiers...