Phantom Ganon, the boss of the forest temple who isn't even remotely forest related. maybe that has something to do with the fact the forest temple was originally supposed to be the wind temple? who can say... either way, Phantom Ganon's battle involves him riding his horse in and out of paintings. the poe sisters, whom I previously covered, also were found in paintings. you may be wondering why the forest temple and its bosses are so painting focused, but there is a reason for that.
Mario and Zelda, having the same creator, share a lot of DNA. it's common for an idea that started in Mario to end up in zelda, and vice versa. in this case, during Ocarina of time's development they weren't sure if they could create a big, connected 3D world. while the Nintendo 64 was set to be the MOST POWERFUL console at the time, the fact it used Cartridges instead of Disks limited the maximum storage space games could have on the console.
at one point, they considered having all the game's locations connected via Ganon's tower, and Link would access these locations by JUMPING INTO PAINTINGS. that's right, just like Mario 64! Phantom Ganon's battle comes from an early idea for the final boss fight, in which Ganon would ride in and out of the paintings. of course, they didn't end up going with the painting Idea and ended up being able to create Hyrule field. this idea for a final boss based around paintings likely would have gone unused, but they repurposed it into a mid-game boss fight by making a fake Ganondorf.
unlike the real Ganondorf, whose appearance changed a bit during the 7-year time jump, Phantom Ganon initially looks identical to the Ganondorf of 7 years ago, the Ganondorf Link remembers. but then he puts on a spooky skull mask so you know he's a phantom.
there were actually more ideas for the final Ganondorf boss battle that didn't make it into Ocarina of time in any form. these ideas came after the painting idea, as both utilized hyrule field. one idea was to have link battle Ganondorf on horseback through hyrule field, an idea we wouldn't see come to fruition until Twilight princess's final battle. the other idea was to have Ganon's pig form be massive and you had to climb up him shadow of the colossus style, an idea we wouldn't see until... megaleg from super mario galaxy? yeah, according to interviews, that's what became of that concept. like I said, mario and zelda share ideas all the time.
Phantom Ganon would appear in later games fulfilling the same purpose of letting the player fight A Ganondorf before fighting THE Ganondorf. he appears in wind waker, where he doesn't even remotely resemble Ganondorf. he then appears several times in Tears of the kingdom, but I also have heard some people argue that the four blight Ganons from Breath of the wild count as phantom Ganons. those people are wrong. those are not phantom Ganons; they are blight Ganons.