One of the 4E Dungeon adventures had an encounter like that, with a pool that would randomly change the elemental affinities of monsters. I forget whether the monsters were "mephits" or generic elementals. The Den of Dreus, a sequel to HS1 - The Slaying Stone.I kind of like the idea that a mephit changes type when exposed to a different element, and that could be the source of some interesting encounters or adventures.
How would a storybook gargoyle act? Limned in faerie lights, guarding sacred forest groves, or cavorting with will-o’-wisps in swampy ruins, a “feygoyle” could be great fun. It might prefer a game of riddles over combat, or offer intruders a suspicious invitation to tea.