Green Bubble

The Green Bubble appears in various dungeons throughout Ocarina of Time. They float in a set pattern, with their green flames acting as a barrier. To defeat them, simply wait until their flames die down, or put them out with your shield, then attack them. They won't attack directly, and so can be considered easier than most. They often appear in groups of two.

One appears floating around the water way in the Kakariko Well mini-dungeon. This one is larger than usual, but still as easy to kill.

Trivia

 * A similar creature named Bonehead appears in the N64 Game Kirby 64: The Crystal Shards. Both monsters share a few notable similarities including having wings and growing the wings when it is approached.