I don't think Tonberries have anything to do with will-o-the-wisps. I have a big, thick book on ghosts and paranormal entities in my bedroom, and I do not see any correlation between wisps and Tonberries. Tonberries are made up creatures while will-o-the-wisps are known to actually exist. Will-o-the-wisps are essentially occurrences of spiritual energy drawn by the Earth's own magnetism, and are given a visible form (usually light). You are more likely to find real wisps in areas with a lot of history and death, such as Gettysburg and battlefields. Sometimes wisps can actually be elementals, and that was touched up on an episode of Ghost Hunters are year or so ago on SyFy (TV). I have seen a wisp myself when I was in Williamsburg, Virginia. It was October, so no fireflies were out, but I saw a flash of light occur across the field when doing a ghost tour; on the other side of the field was a salt marsh, so I don't think anyone had a flashlight!

http://en.wikipedia.org/wiki/Spook_Light

This is the most well-known one here in the US, and it is said to actually be real and has VERY intelligent behavior; if it appears, it can follow you around, examine you, disappear, reappear a distance away and zoom off. http://en.wikipedia.org/wiki/Hornet_ghost_light