I'm not sure whether this is the oldest example, but probably the most famous ancient (fictional) dog is Odysseus' beloved Argos from the Odyssey. Argos waits faithfully and watchfully through his master's tumultuous, well, odyssey. By the end of the tale, he is so old and worn out that he can't do much more than hang around, but he must keep the vigil. Odysseus, of course, has to return home in disguise for fear of his life. The one creature who recognizes him? Loyal Argos, who joyfully realizes that his master has returned. His vigil is finally ended; his purpose fulfilled. Argos can (and does) die at peace.