A Shadow Ruto, which he defeats.

...a shadowy form, entirely black, one that resembled in every way the body that Link now possessed, Ruto's body. It looked at him with empty, red eyes, devoid of compassion or empathy, and it started to walk towards him. To Link's surprise, his heart beat quickly in flighty terror, and he instinctively stepped back for the creature walking towards him. "R-Ruto?" he asked, his voice high and thready. "Is that y-you?"

The creature continued stalking towards him, and when it was close enough it used it's fins as blades, the same way Link had, and began to attack. Using some remnant of his former body, Link managed to dodge the attack and deliver one of his one, but the shadow Ruto dodged away from the strike. They looked at each other, Link finally prepared and ready for a fight, when it spoke back. The voice it used was similar to Ruto's, but as if being heard from a long way away, an echo of its original self. "You stole my body."

Anger pulsed through him at those words, and before the shadow doppleganger was ready to strike Link rushed forward and attacked with more speed and grace than he thought he was capable of. "I didn't want this!" he heard himself yelling, landing a blow on the Dark Ruto's arm. The cut didn't seem to do much to the creature, and within seconds it was returning the attack. They exchanged blows, each managing to land small cuts and glancing strikes at each other, when finally Link noticed that the beast's guard had dropped slightly, and rolled around to it's back and delivered a jumping strike that split the thing from bottom to top.

Dark Ruto dissolved before his eyes, fading away into the aether. "You're a fool, Princess."

With that done, Link saw the door unlock, and continued towards it, completely ready to get what he needed and leave, to forget that horrible fight. He wasn't Ruto. He wasn't.

Inside the next room was an upgraded Hookshot, the Longshot, and with it traversing the temple became much easier. Within an hour Link stood ready to fight Morpha, the evil creature corrupting this temple.