"Once Olaf was in Ireland on some warlike expedition with his fleet. And when they required to make a raid on the shore for food, some men went on land and drove a great number of cattle down to the shore. Then a farmer ran after them and prayed Olaf to let him have the cows he owned, and Olaf told him he could have his cows if he recognized them, "but don't delay us." The farmer had with him a large cattle dog. He pointed him into the flock, where many hundred cattle were being driven. The dog circled the whole herd and drove away as many cows as the farmer said he owned, and they all bore the same mark, so they believed that the dog had recognized them properly, and they thought the dog marvelously clever. Then Olaf asked the farmer if he would give him the dog."Gladly," said the farmer. Olaf right away gave him a gold ring in return and promised him his friendship. That dog was called Vigi and was a most outstanding dog. Olaf kept him for a long time afterwards."
-- from the Saga of Olaf Tryggvason, in Snorri Sturleson's Heimskringla (c. 1230)