Final answer:
The treacherous goatherder in 'The Odyssey' is Melanthius, a character contrasting with the loyal Eumaeus.
Step-by-step explanation:
The treacherous and opportunistic goatherder in The Odyssey who supports the suitors and abuses Odysseus without realizing his true identity is Melanthius. Unlike Eumaeus, who is loyal to Odysseus, Melanthius is depicted as a villain who is disloyal and abusive. His character is an example of those in Ithaca who have not remained faithful to their absent king, Odysseus.