Gleeok

Gleeok is a boss monster appearing in multiple games. It is a dragon with multiple heads, and if the necks are struck enough times the heads detach and fight independently until the body is finally killed.

The Legend of Zelda
Gleeok makes three appearances in the first game. All are defeated in similar fashion. Using the sword, Link must cut off each of the heads on the beast. Once a head is detached from the body, it will float around the room shooting fireballs at Link. He must avoid the fireballs and cut off the other heads; finally, Link may collect the Triforce piece.

A two-headed version of the beast guards the fourth piece of Triforce in the Snake Labyrinth, a three-headed version is a mini-boss in the Dragon Labyrinth, and a four-headed version of the beast guards the piece of Triforce found in the Lion Labyrinth. In the Second Quest, there is an two-, three- and four-headed versions guarding pieces of Triforce in the Moon, Lizard and Demon Labyrinths, respectively.

It is possible to defeat a Gleeok in one hit with a well-placed bomb.

Oracle of Seasons
The Gleeok within the Oracle of Seasons is a two-headed version fought in the Explorer's Crypt. It is defeated in a different fashion from its former counterpart. Getting rid of the beast's heads is still Link's main objective, however, there is one more step before defeating it. Link must slash away the head, which is a harder task. Once the heads are detached, the beast will start jumping, causing a massive earthquake in the arena. Using the Roc's Cape is the only way to avoid serious damage from the beast. When the beast is grounded Link should attack with the sword. He must continue this process until he is defeated. He will then give up the Seed of Life, the seventh Essence of Nature.

The Minish Cap
Though "Gleeok" is not seen in The Minish Cap, the boss Gleerok is a one-headed Gleeok with a rock on its back. This information is from the figurine.

Phantom Hourglass
Gleeok reappears in Phantom Hourglass as the Two-Headed Dragon Gleeok, boss of the Temple of Ice and guardian of the Azurine Steel. One of his heads is red and the other one is blue. The red fires a ball of flame and the blue shoots a ball of ice.

Link will be standing on a platform with four pegs near each corner. When one of the heads looses a blast at Link, he will need to attach the grappling hook to two pegs diagonally so as the rope launches the blast back at the other dragon's face, cracking it's mask. In addition with these blasts, the two heads will lunge to snap the rope Link ties, while trying to bite him as well. They can also sink below the water to cause a tremor that will flood Link's platform, as well as cause chunks of ice to fall from the ceiling and crash into Link. Link can tie the grappling hook to two posts and walk along the rope to avoid the ice chunks. Three blasts to both heads will remove their masks.

Both heads will remove two of the four pegs by snapping them with their jaws. Each head will take turns at trying to bite Link, then will charge up for a powered blast. The red dragon head will loose a fiery blast that will scorch him, as well as leave a trail of fire on the platform. The blue dragon head will loose a blast of ice that will freeze Link on contact. While charging, however, both dragons will be sucking in energy, exposing their tongues. Link can pull the head down by tying one end of the grappling hook to a post, and using it as an anchor on the dragon's head. Once down, Link must slash at the dragon head before it recovers. Repeating this process to both heads will defeat the dragon, and Link will obtain a Heart Container, extra sands for the Phantom Hourglass, and the second of the three Pure Metals, Azurine.

Skyward Sword
Gleeoks appear as a common enemy instead of a boss in Skyward Sword in the Skyview Temple and the Lanayru Mining Facility. Link must use his sword to cut off all the heads at once, as they will grow back if he doesn't.

Animated series
Gleeok appears in two episodes of the animated series. In "Kiss'n Tell", Ganon sends a Gleeok and a Gibdo disguised as a beautiful maiden to a forest, where the Gleeok pretends to be attacking the maiden so that they can lure Link into the scene. However, Princess Zelda arrives instead and proceeds to try and fight off the Gleeok by throwing various kinds of fruits at the three-headed monster. When the Gleeok is stunned for a moment, Zelda sees a chance to go but the maiden refuses, saying she wants to be rescued by a handsome hero. Soon after, Link appears and battles against the Gleeok, who quickly zaps the two outer heads. With only one head left, Link grabs the Gleeok's tail, but the monster begins to run around in circles. Zelda, who was eating a banana from the fruit basket, throws the banana peel to the ground and causes the Gleeok to slip, allowing Link to deliver the final blow and send the Gleeok back to the Evil Jar.

In "Hitch in the Works", when Link arrives to the Underworld and tries to stop the wedding between Princess Zelda and Ganon, the evil wizard summons a Gleeok from the Evil Jar to attack Link. Link fires three Sword Beams at the monster's necks, but the heads separate and come after him. He skillfully dodges their attacks, causing them to fly straight into Ganon, thus destroying the evil wizard and the remains of the Gleeok.

The Legend of Zelda comic
A four-headed Gleeok appears in the Choices story of the Legend of Zelda comic. Link encounters it as he is traveling through the Underworld in search of Princess Zelda. Although Link at first seems to have trouble fighting the Gleeok, Link is able to cut off all four of the Gleeok's heads with a little help from Miff. The disembodied heads then begin to attack the young hero, but after some struggle, Link is able to defeat all of the Gleeok's heads.

A Link to the Past comic
In the A Link to the Past comic, Link is attacked by monsters called Gleeocs in the Palace of Darkness. However, the monsters are bright orange lizards with green gems on their heads, and thus physically resemble  smaller versions of the Helmasaur King which is fought in the game. After Link is discovered wandering around the palace by a Rocklops, he is attacked by a Gleeok  that was called upon by the Rocklops. Although he defeats it easily  with the Master Sword, soon more Hinoxs riding Gleeocs begin to head   towards him. Link then gives Epheremelda a bomb, which  eventually denotes the firecorn stockpile and destroys the entire   palace and, with it, the Gleeoks.

Super Smash Bros. Brawl
Gleeok appears as a sticker in Super Smash Bros. Brawl.

Trivia

 * A three-headed Gleeok also makes an appearance in the "Kiss 'N Tell" and the "A Hitch in the Works" episodes of The Legend of Zelda TV series.
 * Gleeok without his masks in Phantom Hourglass bears a striking resemblance to Chilly Willy and Chilli Billi from the Banjo-Kazooie series. However, this is likely a coincidence because Nintendo doesn't have rights to Banjo-Kazooie anymore.
 * Gleeok is the only boss from the original The Legend of Zelda to not appear in its Broadcast Satellaview remake.
 * The masks these creatures wear in Phantom Hourglass are similar to that of the Helmaroc King. Without their masks they also bear a striking resemblance to Valoo.
 * Being a multi-headed dragon, Gleeok bears similarities with the chthonic water beast of Greek mythology, Hydra.