In the local community where I was born - Orkney (small group of islands off the north coast of Scotland for those who don't know) - there is current usage of thee, thou and thoo. There doesn't seem to be a hard and fast rule, but generally thoo is the plural form.
I'm not sure of the historical root, but Orkney was a Danish Viking colony, only sold to Scotland recently (a few hundred years ago) as dowry so it may be a merging of Old Norse and Old English.