Will-o'-wisp covers both ancient fokelore and natural recorded phenomenon. This would be a great eerie addition to the the Swamp.
Have them appear in minecraft as balls of light that travel swamps at dusk/dawn, then disappear suddenly. They could be in the shape of small glowing balls (or blocks) that just wisp around the swamp.
Are they an actual animal? Are they a mob at all? Or is it just something weird that happens with swamp water? It would be up to the player's imagination to decide!
But if you bat them down with a silk-touch shovel, then they would drop something. Or perhaps you can catch one in a bottle. Exactly what they do I'll leave to the discussion.
(I separated this from the Swamp thread since the feedback team is preferring we do it this way.)