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 magic, 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 magic, 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.
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.
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.
Battle with the Thunderbird from The Adventure of Link
↑ "The enemies in the Palace are those that were made and chosen by the King. They're abnormally strong." (The Adventure of Link manual, pg. 38)
↑ "When you have set crystals in all of the statues in the six palaces, the 'binding force' placed on the Valley of Death will be removed and you will be able to enter the great palace. There you must fight the last guardian. And you can obtain the Triforce only by defeating that guardian." (The Adventure of Link manual, pg. 11)
↑ 3.03.1"Master... The people are restless! Is it wise to let them see this?" (The Legend of Zelda (Valiant Comics), pg. 5 [Coming Home])
↑"I was set upon and defeated by someone who looked exactly like me AND had all my powers. Every magic spell I used--it threw back at me! Now this...thing, disguised as YOU, Link, has taken over the kingdom." (The Legend of Zelda (Valiant Comics), pg. 4 [Coming Home])
↑"The trial continues until only one survives!" (The Legend of Zelda (Valiant Comics), pg. 8 [Coming Home])