WARNING: POTENTIAL SPOILERS
I believe the Night King in the television show Game of Thrones is Rhaegar Targaryen. This is all speculation (obviously) and I will only cite subjective evidence from the show, not the books. We know that the show is now splitting from the direction of the books, so we should only consider what we already know based off of the show.
Here are six reasons I believe this.
He looks kind of like a dragon, right?
He can walk through (ordinary) fire.
He hones in on Jon Snow specifically whenever he’s around.
He takes only male infants from Caster, then cradles them – possibly recreating the moment he never got to have of holding his newborn son (Jon Snow).
The timing is right – it’s taken this long to build up his armies.
It would make sense that he’s Targaryen, because dragon related things kill him (and others like him). His living side is his weakness.
We saw the night king get created by the Children of the Forest, and this was thousands of years ago. But there’s nothing saying definitively that this Night King is the one was saw in the flashback. There could be several. That one could have been defeated in the first battle, the battle documented in the caves at Dragonstone. And we know more White Walkers can be created – not all of the army consists of zombies, some are sentient warriors.
Maybe Ned spared Rhaegar and condemned him to a life beyond the wall. Maybe his body wound up there through other circumstances. This would also explain why he has it out for Bran – he would have a grudge against the Starks for killing him.
Or I’m totally wrong. 🙂