I recently read a blog post on the Anytime Yoga blog musing over the dual nature of the word "fetching." One commands a dog to "Fetch!" One also describes a pretty woman as "fetching." Seems a bit insulting, doesn't it?
Both senses of the word "fetching" have a common origin in the Proto-Indo-European term for "foot." It is a cognate with the Dutch, German, Faroese, and Icelandic words for "to grasp, understand, and bring." In other words, it is at its origin a word about attraction, grasping, holding… and feet.
This seems a bit odd to us now, but I'm sure it made more sense a few thousand years ago, when the only way to get something (or to have it brought to you) was by foot. Either yours or someone else's. Or those of an animal, which is no doubt where we get the sense of the term "fetch" as a game played with a retrieving dog.
Beauty has long been seen as a magical thing, something which exerts an unusual power over other people in the vicinity. And this is where the use of "fetching" to mean "a pretty girl" is probably from. Think of those old Warner Brothers cartoons, when (say) Bugs Bunny swoons over a beautiful woman, and finds himself being pulled towards her by a magical force. Drawn in by her beauty.
It is this "drawing in" that gives us this sense of "fetching."
As befits such an old word (one of the oldest), there are many uses. To travel by a sailboat is to fetch. You can also fetch a nice price for an item on eBay. It's easy to see how these uses of the word came from the original meaning of movement and bringing something towards yourself.
I was able to find less information on the use of "fetch" to mean a wraith or spirit. The ancient Germanic people called ghosts "fetches," with a heavy emphasis on the idea that a fetch was a doppelganger or guardian spirit. Perhaps this meaning comes from the mystical pull that such creatures exerted over their victims. Although most of us would be inclined to run the other way, a fetch was notoriously fond of luring people towards it (so that they could be killed, or at the very least frightened gray-haired overnight).
Interestingly, some cultures place the agency upon the human. For example, the Norse believed that these spirits followed the man who saw them, rather than the other way around. Perhaps it is us fetching the ghosts, and not the reverse!