Before Odysseus left he had a dog named Argos. But, after he left they didn't take care of him and because, Odysseus was gone for so long, Argos was dying.
So, even though Odysseus was disguised as a beggar.When Argos heard Odysseus's voice he knew who it was. He struggled to wag his tail. According to Book 17 it says, "But when he heard Odysseus's voice nearby, he did his best to wag his tail, nose down, with flattened ears, having no strength to move nearer to his master.
Then sadly that was his last happy moment before he died.