Vitreous

Vitreous is the boss of Misery Mire and captor of the sixth Maiden in A Link to the Past. It is a gigantic eyeball surrounded by smaller eyeballs, immersed in a green liquid.

Strategy
The main eyeball will shoot lightning at Link, similar to one of Agahnim's attacks, while the smaller eyeballs will attempt to swarm him. To avoid this, stand in one of the corners of the room and use the spin attack. Once the smaller eyeballs have been taken out, the main eyeball will attempt to charge at Link. This giant, massive eyeball can be defeated with sword strikes, but takes more damage from arrows. Defeating Vitreous earns Link a Heart Container and the sixth imprisoned Maiden.

Alternatively, Link can also use the Magic Cape to dash right into Vitreous and slash him until he is defeated.

A Link to the Past comic
Vitreous makes an appearance in Chapter 9 of the A Link to the Past comic. After Link falls for the Wizzrobe's trap by heading into Misery Mire, the Wizzrobe conjures Vitreous from the swamp to devour him. However, as Vitreous approaches Link, a whirlpool is created that engulfs both Vitreous and the Wizzrobe, and from beneath Zora appears to rescue the young hero.

Trivia

 * The name "Vitreous" comes from Vitreous humor, the clear gel found inside the eyeballs of vertebrates.
 * Vitreous is called "Gerudōga" (Geldogger is the rough translation) in Japanese. The suffix -dogger, in addition to its eyeball appearance, connects it to Digdogger, a recurring boss in the Zelda series. The prefix gel- is probably used due to the fact Vitreous is immersed in a green liquid.