A faintly-glowing sphere of light easily mistaken for a lantern
A Will-o'-Wisp is a faintly-glowing sphere of light that is yellow or white. It is easily mistaken for a lantern, especially in the foggy marshes and swamps where it typically resides. A will-o'-wisp's body is a globe of spongy material about 1 foot across and weighs about 3 pounds. Its body casts as much light as a torch. It has no vocal apparatus, but can vibrate to create a voice with a ghostly sound.
Basic Fantasy Field Guide Omnibus