Gekko is a mini-boss from the Woodfall Temple and the Great Bay Temple in Majora's Mask. Despite their name, these enemies are frog-like orange, which is partly because they are both frogs from the Frog Choir who were cursed by evil from the temples they are found in.
The Gekko in this temple is not alone. It rides on the top of a Snapper and must be knocked off the back of the Snapper to be weakened. Once the Gekko is off the Snapper, Link must hit it with an Arrow as it is running around the room to hurt it. This process must be repeated until the Gekko is defeated.
To get to the Gekko, enter Woodfall Temple and use the deku flowers to get to the door straight ahead to the north. In this next room, jump down to the lower level and take the door to the right. Shoot an arrow at the eye to the left, then take the deku flower to fly to the northern door. The Gekko is beyond this door.
Great Bay Temple
The Gekko in this temple is also aided by another enemy, Mad Jelly. When Link first enters the room, the Gekko will drop Mad Jelly from the ceiling. He walks around in the jelly, once in a while picking one up and throwing it at Link. After the Gekko is damaged, it collects all the Mad Jelly into one glob and surrounds himself with it on the ceiling. If the Gekko drops the jelly ball on to Link, he will become stuck and the Gekko will begin to punch him until he is out of the jelly. Link must freeze the jelly mass with an Ice Arrow, causing the Gekko to crash to the floor with the Gekko vulnerable. The Gekko will pick itself up and begin throwing the Mad Jelly at Link again, starting from the beginning. If Link continues to hurt the Gekko then freeze the jelly blob, he will eventually defeat the Gekko and be rewarded with the Boss Key.
To get to the Gekko, enter Great Bay Temple and make your way through the door to the north.Jump into the whirlpool and there will be two possible routes to take. Take the path closer to the surface, then continue through the underwater path. Make your way through the next hallway, then enter the door. There is a passage across the water that requires the use of ice arrows to access. Use ice arrows to create platforms to get to it, knock down the icicles that block your way, and use a fire arrow to melt the ice at the end of the hallway. Enter the door, where you will find the Gekko.
Gekko and Snapper from Majora's Mask.