Did the previous owners ever got any threats from the original owner number one? If not, then, she is taking advantage of OP's horse. I am certain if they wanted the horse that bad, they should have reported it and recovered the horse alot sooner than that.
Sometimes registries will take the first owner's name if you got it, as the last known owner and sometimes it is a clue unless she has several horses of the same parents as your mare.
Sometimes registries will take the first owner's name if you got it, as the last known owner and sometimes it is a clue unless she has several horses of the same parents as your mare.