Gleeok

Gleeok in The Legend of Zelda
The Gleeok makes three appearances in the first game. All are defeated in similar fashion. Using the sword, 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. Avoid the fireballs and cut off the other heads and the Triforce piece is yours.

A two-headed version of the beast guards the fourth piece of Triforce, a three-headed version is a mini-boss in the sixth underworld, and a four-headed version of the beast guards the final piece of Triforce

Gleeok in Oracle of Seasons
The Gleeok within the Oracle of Seasons is a two-headed version. It is defeated in a different fashion from its former counterpart. Getting rid of the beast's heads is still the main objective, however, there is one more step before defeating it. Slashing away the head is a harder task, more slashes of sword will detach the heads. 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 attack with the sword. Continue 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 Gleeok with a rock on it's back. (This information is from the figurine.)

Phantom Hourglass
Gleeok reappears in Phantom Hourglass as the 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. To defeat it you need to use the grappling hook on two diagonal pegs to bounce back the attacks of one onto the other. If one is hit by it's own attack it will be unaffected, but being hit by the other will cause it's mask to crack. After doing this, the helmets on the heads will get destroyed and 2 pegs will be destroyed by Gleeok. At this point the heads start coming closer to shoot at Link, and their tongues will stick out. Now Link has to use one of the remaining pegs and use the grappling hook on his tongues to bring down the head and start slashing away with the sword.