The Grangalan enemies? The smallest ones are called Grangalan Jr. Jr. and drops the least gil, the middle ones only have one Jr. in their name and drops a bit more, while the big one is simply Grangalan and drops the most. So while they don't drop a lot of gil, the "older" ones drop more.

Or one could reason another way and say that if you kill the Grangalan before it reproduces it doesn't give a whole lot, but if you let it reproduce and then kill the offspring, you'll get a good deal more. That way it "improves with age" too, in a way.