Goeran (_Vintersolstaandet_) sees the wild hunt, or rather Oskoreien, as a phenomenon which is closely connected with X-mas/winter solstice: as we know, the dead return, on the longest night of the year, to the place where they lived - this is also known from Finnish and Celtic beliefs - and this, says Goeran, is what the wild hunt is about.
This is not all Goeran has to say, but I dare call it his most important hypothesis of the wild hunt.
If we are to test it, we should ask "how often is the wild hunt bound up with the longest night, and how often is the belief of the return of the dead at the longest night bound up with the wild hunt?"
The answers are: "outside certain parts of Sweden, rarely and rarely." Since the belief in the wild hunt is not very widespread in Sweden, the logical conclusion has to be that the widespread belief in the return of the dead on the longest night has attracted the - apparently rather exotic - wild hunt. Both deal with collectives of ghosts - a phenomenon otherwise very rare in Scandinavia, except for the Hiisi of southern Finland. The only other instance I can think of where collectives of ghosts appear in North European (minus Finnish) folk beliefs is the "mass of the dead." Thus, it looks like Goeran's hypothesis merely holds for a case of secondary attraction.

I once tried to dig down to the roots of the "everlasting battle." In the Nordic material, this legend is usually termed the "hjadningavig" (sorry, my email Brownie doesn't accept diacritics), a name which is so similar to the Finnish "hiidinvaeki" that it ought to have made scholars suspicious. The idea of the everlasting battle of course do not come from the Finns, only, when it reached Scandinavia, its "swarm of dead" was so foreign that they were apparently identified with the only "swarm of dead" already known in the North, the Finnish/Karelian hiisi.
After digging my way back through Ibn Fadhlan (mentioned below), Damascios (c. 525, everlasting battle with Attila's Huns in Italy) and Pausanias (second century, battle of Marathon repeated yearly), etc., I gave up on the everlasting battle. I simply wound up with what is, in Danish, called "gammel faerd", "genfaerd," i.e., the idea that ghosts re-enact an event, be it an army doing battle, or a white lady taking a walk through a wall where there once was a door. This is what ghosts generally do in Eurasian folk beliefs, and probably also in the rest of the world.
In Europe, supernatural beings - including ghosts - are usually solitary beings; only few cultures, like the Finno-Ugrian, know swarms of dead. Thus, when we encounter collectives of ghosts outside these cultures, they usually have a reason for being together: they form an army, or a hunting party, or partake of a hunt. But it may be suspected that behind this lie foreign ideas of the "swarm of dead," which have been assimilated.
I am not, mind you, talking of supernatural beings living in family groups. We know some who do this (alfer/elver/alvar, the Danish bjergmand, etc.), but of large and amorphous collectives like the Finnish hiisi. Such collectives of ghosts are known all other Asia and parts of Russia, but not really further west than southern Finland.

If this holds true, then the wild hunt must also be a foreign idea, since it includes a swarm of supernatural beings, and it may have come to us from Asia or Russia.
The first record we have of anything like the wild hunt is in Ibn Fadhlan's account of his visit to the Volga Bulgars in the early tenth century. They had what we would term shamanist belifs, but wished to become Muslims. In the night, Ibn Fadhlan heard sounds of horses, shouts, and even fighting in the air, and the Bulgars explained to him that it was the spirits of the dead doing battle, good against evil.
There is an element of the everlasting battle here, but there is also what seems to be the kernel in the wild hunt: a swarm of supernatural beings chasing through the air.
We should note that Ibn Fadhlan heard it. Of course he may be lying - he seems to sometimes make his account as colourful as possible - but it nevertheless seems probable that he heard some sort of natural phenomenon which was explained as supernatural beings chasing through the night air. Once such an idea gains popularity, it may of course be used to explain various sorts of sound phenomenae in the night. In Scandinavia, it is quite often bound up with wild geese, but also with storms.

Now let me try to sum up the elements of superstition/legends I have tried to deal with:

1. A collective of supernatural beings making noise flying through the air
2. A collective of ghosts
3. The everlasting battle

It is obvious that all three elements _may_ appear in what we call the wild hunt.
It is equally obvious that only 1. is obligatory. This is what the wild hunt _is_.
2. is an extra. Odin and his hunters are not ghosts, and neither are frau Holle or Perchta and their followers.
3. is not even very common in the wild hunt, but may appear.

I therefore conclude that the only stable trait in the belief/legend of the wild hunt is 1. This brings us back to the time-honoured view of folklorists, that the whole thing is of an etiological nature: supernatural phenomenae (=inexplicable natural phenomenae) are caused by supernatural beings.
Thunder (=Thor-doen) is made by Thor, the flickering of flames and other light phenomenae are made by fickle Loki (=flame?), the music in the waterfall by noekken/naecken (=O.E. nicor, mod.E. nix), etc.
The only problem is: which natural phenomenon exactly is it that is caused by the swarm of supernatural beings in the air? -hardly wild geese; people would know wild geese, and the attraction of the legend/superstition to them seems secondary.
I think the phenomenon is one which is rare in these parts of the world, but which occurs more often in large river valleys. I have experienced it only once, at the Wistula many years ago. It came rather suddenly; two strong gusts of wind that sent me swerwing on my motorbike and had me stop and dismount under a large tree. Then something passed which sounded like an express train and a hundred howling demons. Blew my motor bike clean off the road, threw me to the ground, and left me saying "this was Odin's hunt!" Took about one or two minutes. First thing I saw when I continued was a thatched roof on the road. A little further on there were some people standing around a house with no roof on it. I have been told that the phenomenon is rather common at the great rivers in the steppe.

To people who believe in swarms of spirits it is rather natural to explain such a phenomenon - which sounds like there is a lot of demons or whatever involved - as the passing of such a swarm. However, to people who do not hold such beliefs, it will be necessary to explain what caused the swarm to gather. Thus, it becomes an army, or a hunting party, or the followers of a prominent ghost. This secondary explanation may vary, not only in space, but also in time. Here in Denmark, it is either Odin ("Goen," "Joen Opsal," etc.), or "king Valdemar." There were three Valdemars, but folk traditions do not care about such trifles. But more of that later if anyone is interested; this has run extremely long already.

Lars Hemmingsen, Ph.D.
Department of Folklore
University of Copenhagen