I have made what is essentially raw hide out of beaver tail. I soften it in warm water, then form it around whatever it is to be used for (bow handle in your case). It is also best to stitch together while still soft and then let it dry on the bow, it will shrink some which will pull it tightly against the handle section.
Here is ONE way to do this (other people might have a better way):
Split the tail by cutting through the lateral side (parrallel to the flat sides) all the way around. When you are doing this you will feel the bone, carefully cut around the bone and then you can pull the tail apart in two pieces.
Now cut away as much flesh as you can. Then lay the tail on a hard surface and scrape the flesh side until you come down to the inside of the hide (it will be like a waxy leather). You may then want to wash it real well with a degreasing soap, it cleans off much of the grease and takes away any unpleasent smell. Finally if you are not going ot use both halves of the tail right away you should nail them flat ona board to dry. Raw hide will last essentially forever when stored in a dry and bug-free place.
Good luck, a beaver tail bow handle should be really cool, Juniper Bow