A Guay is a small bird that appears in multiple The Legend of Zelda games. They usually come in flocks, and dive at Link's head to attack. They often leave hefty Rupee awards when either defeating individual Guay, or a large flock.
Ocarina of Time
In Ocarina of Time, Guays only appear at the Desert Colossus, Lake Hylia, and Lon Lon Ranch during the nighttime. Link can use ranged attacks, such as his Slingshot, Boomerang, Hookshot, or Bow to defeat the enemies. Alternatively, a timed sword slash when they dive down also defeats them. After Link defeats a Guay, another one respawns a few moments later and they will infinitely respawn. After defeating a number of Guay, a larger Big Guay appears, which is just as easy to defeat, but it gives an even larger sum of Rupees as a reward.
Guays are sometimes found near tall ladders, including a Guay that appears above the Lakeside Laboratory. A Piece of Heart can be found atop the ladder, but the Guay makes it much more difficult to reach the peak as it flies into Link while he is climbing the ladder. They almost always need to be defeated so that Link can reach his destination.
It's a Guay. You don't know about
In Majora's Mask, Guays are similar in look to the ones from Ocarina of Time having purple-colored bodies and orange beaks. They mostly appear in trees, although there is a single Guay that circles Clock Town in Termina Field, but it does not attack. If Link plays certain songs on his Ocarina while wearing certain masks, the Guay drops Rupees for him. One example of this is if Link plays the New Wave Bossa Nova while wearing the Zora Mask. Guays can also be found near the pond in Snowhead during Spring, and also in Ikana Canyon.
Guays take on a different appearance in Twilight Princess, looking more like an actual bird and having more of a twisted enemy design. They resemble a vulture and have bright blue eyes. They usually attack in flocks, but they can also be found alone, nesting in trees or on top of rocks. When these enemies attack Link, they continue to attack until Link has run away, or if all of them have been defeated. A large group of Guay can be found in the Graveyard at Kakariko Village and if Link defeats the entire flock, a large sum of Rupees fall from the sky.
Target lock: Guay
In Skyward Sword, these small bird-like enemies dive-bomb Link if they spot him. When these enemies dive down toward Link, Link can Shield Bash them, causing them to be stunned and fall to the floor. Link can finish them off with the Fatal Blow. However, if Link doesn't defeat them in time, the Guays recover and take flight once again.
They also have a secondary ability to drop feces upon Link's head. This creates a noxious yellow cloud that drains hearts until he shakes it off. When the Guay is defeated, it drops either 5 or 20 Rupees.