I've never been able to discern any kind of pattern to their spawn order, except that they seem reluctant to appear in a location if you're standing there waiting for them. But even that doesn't hold true all the time—sometimes they're not shy at all. If there is a pattern, it's probably tied to the game clock somehow, like so many other semi-randomized things. I doubt this would have been changed among the various ports, as it seems like a pretty obscure thing to want to change, and no real benefit for doing so. I could be totally wrong though.
The one consistent thing I've noticed is that a Nu will not reappear in a particular location after it's been defeated there. However, it seems that the game can still randomly assign that location as the spawn point every time it rains, meaning sometimes no Nu will appear on the whole map after you've defeated one. The problem worsens after you've beaten two Nu, and after three, you'll have to leave and re-enter the area to get them to spawn.
