Thunderbird

The Thunderbird is the primary guardian of the Great Palace and the penultimate boss in The Adventure of Link. It is a bird-like creature that was placed there as one of the many tests prepared for the hero seeking to acquire the Triforce of Courage by the ancient King of Hyrule that once ruled over the entire kingdom with the united Triforce.

Strategy
When Link enters the chamber of the Thunderbird, it will not be anywhere to be seen at first. However, after a few moments, it will swoop down upon Link from above and proceed to attack him with fireballs. It will have a red color to it and will at first be invulnerable to attacks, leaving it invincible at first. To reveal its weakness, which is its face, Link must cast the Thunder Spell, which he can learn from an old man in the Old Town of Kasuto after attaining the maximum level possible for his Magic Meter. Casting this spell will cause the Thunderbird's color to change from red to blue and will make his face appear, leaving it vulnerable to sword attack. Link can then slash the beast's face with his sword, inflicting damage upon the palace guardian. In response, the Thunderbird will quicken its pace of throwing fireballs, which can only be repelled with the Reflect Spell, at Link.

Once Thunderbird has taken enough damage to its face, the Thunderbird will disintegrate, leaving behind the key to the palace's final chamber, where Link will face his final test to prove his worthiness of possessing the Triforce of Courage.

The Legend of Zelda comic
Thunderbird also makes an appearance in the Coming Home story of the Legend of Zelda comic. Unlike the game, Thunderbird is much smaller and somewhat resembles a Moby. He seems to be a servant to Shadow Link, who he calls "Master." In the story, Thunderbird accompanies Shadow Link in defeating Queen Seline and taking over Calatia. Although Thunderbird does not play a major role in the story, he does advise his master about not letting the Calatians witness the fight between Shadow Link and his good counterpart, but later he is the one that signals the start of the fight between the two. After Shadow Link is defeated, it is unknown what becomes of Thunderbird.

Trivia

 * His name could be a reference to the mythical Thunderbird, a giant bird who kidnaps children. Note that the Helmaroc King from The Wind Waker acts like the mythical Thunderbird since he kidnaps female children.
 * Like Gohdan in the Tower of the Gods in The Wind Waker, the Thunderbird is the guardian placed to test the courage of potential heroes.