Golden Monsters is the general term for the many golden enemies that appear in Oracle of Seasons and The Minish Cap, usually triggered by the activation of an event, or Kinstone Fusion, as in The Minish Cap. These enemies are much more stringent versions of the common Octoroks, Ropes, Moblins, and Tektites, just to name a few. While these incarnations are much stronger, and much more prized by the community, the Golden Monsters in The Minish Cap also see an increase in speed over their lesser, basic variations.
In summer, the water around Gnarled Root Dungeon recedes, revealing a cave with a golden-robed Old Man inside. After Link talks to him, four different Golden Monsters will appear around various regions of Holodrum. When all four have been defeated, Link can return to the old man to receive a Magic Ring which, when appraised, becomes the mighty Red Ring.
In The Minish Cap'sHyrule, three types of Golden Monsters appear after certain Kinstone Fusions. They are Octoroks, Tektites, and Ropes. They are much faster, can take more damage, and dish out more damage, but their basic attack pattern remains the same. After one is defeated, they will drop a large sum of Rupees.
↑"Defeat the four golden creatures and talk to the old man near the Gnarled Root DUngeon. He'll give you a sword-power doubling ring." (The Legend of Zelda: Oracle of Seasons and Ages Official Nintendo Player's Guide (Nintendo Power), pg. 17)