Sea of Thieves added pet monkeys and parrots when its Pirate Emporium went live back in September. The only issue was that pirates had to pay real-world money to buy those pets (and then lovingly fire them out of cannons). As of today, however, you can get a pirate pet for free if you're a Twitch Prime member through your Amazon Prime membership—which costs $13 per month or $120 per year. So... not exactly free, really.
But if you're already shelling out that monthly money for your Prime membership, an otherwise-free purple monkey awaits you. All you need to do is link your Amazon Prime account to Sea of Thieves by following the instructions here and then log in to the game. Head to the pet chest on your ship or at an outpost and you'll find an Amethyst Soul Capuchin (purple monkey) waiting for you.
When you link your account you'll also receive a Celestial Steed Ship Livery Set with purple and pink sails, a matching paint job for your hull, and a figurehead that looks like a unicorn. There also three new emotes with a Twitch theme—one of them is called Let's Go! (of course) and features your pirate thrusting their fists in the air. The other two are an embarrassed facepalm and a gesture that pays respects (sarcastically?) to a fallen fellow pirate.
The fine print says it may take up to 72 hours for the items to appear in-game, though mine were there immediately. The offer is good until January 21, 2020. Should you ever cancel your Amazon Prime membership, you'll still keep your monkey, sails, and emotes "subject to any Sea of Thieves rules or policies that apply to the content," according to the offer.