I'd like to share some links to some great research about the history of Shetland sheep and the term "Beaver coats". Fellow Shetland breeder and friend, Theresa Gygi of Under the Son Shetlands has spent countless hours researching and with some help from another fellow breeder and friend, Cyndee Wolfe of Lacien Farm.
Here is the article that resulted from that effort:
The Historical Use of the Word Beaver
Shetland Breed Information and Terminology
There is a ton of great information here, along with her sources and references. If you want more information and are hungry for research, this will help you.