Glowy butterflies can spawn anywhere, just like all the other collectibles. In fact, I read one report somewhere from someone who had captured a Glowy butterfly at the frog pond in France! This pond is surrounded by about a dozen spawn points, and one of them just happened to cough up a Glowy butterfly.
(Note that I have heard a few people say that they found it "at night." Does this butterfly only spawn at night? Not sure, but it's worth remembering!)