This post contains spoilers from the Game Of Thrones Season 7 finale. Let's not dance around it. Game of Thrones has been dropping book spoiler bombs all season, but none were as big as tonight's revelation of Jon Snow's real name. But with this revelation of what Rhaegar and Lyanna meant to name him comes new questions. What does Aegon Targaryen mean, and why does it matter that this is Jon's real name?
Well, first of all, let's get pedantic. Jon Snow isn't just the first Aegon Targaryen. He's the 11th son of the Targaryen line to bear that name since the Doom of Valyria. The original Aegon Targaryen was the son of Daenys Targaryen, who foresaw the Doom, and convinced their family to evacuate. He was Lord of Dragonstone prior to the conquest.
Of the other ten Aegons who followed him, from Aegon I Targaryen, who lead the conquest of Westeros in 2BC to Aegon Targaryen, son of Rhaegar and Elia Martell in 282 AC, five have been crowned King of Westeros: Aegon the Conqueror, Aegon the Usurper and Aegon the Dragonbane, both who ruled during the Dance of Dragons Civil War, Aegon the Unworthy (who's many bastards beat the Blackfrye Rebellions) and Aegon the Unlikely, also known as Egg from the Dunk and Egg series. That last one was Maester Aemon's brother, by the way.
If anything, Jon is actually Aegon VI Targaryen.
It's a worthy name of kings, but why did Rhaegar insist on naming his son that... especially when (as we just mentioned) he *already* had another son by the exact same name?*
(*This matters more to book readers, because in the novels, that Aegon Targaryen isn't dead, he escaped. The Mountain killed a swapped out baby, and Aegon is about to raise an army and try and conquer Westeros.)
In the TV show, it almost makes sense that Rhaegar would name his new son Aegon, since the Mountain would have just slaughtered his first son by that name. But that's not why he did it. After all, that son didn't matter anymore anyway because Rhaegar's marriage to Elia was annulled, and therefore that Aegon Targaryen was now effectively a bastard. (Damn, sorry dude-in-the-books. You know, the name Jon Snow's about to be going free...)
No, Rhaegar named his son that because his father Aerys had spent a lifetime drilling into him that from their line would come the Prince That Was Promised. And while some might have looked at their child at birth and wondered "Huh, will he be the PtwP?" Rhaegar was egotistical enough to assume any legit male heir that sprang from his loins was automatically the PtwP.
And he was going to name the Prince that Was Promised Aegon Targaryen, after the conqueror and founder of the Targaryen dynasty in Westeros.
So when his first son 's legit birthright was annulled, that just meant "Oh sorry kid, you're not the Prince That Was Promise, my bad! Imma give your name to my new wife's kid. No offense."
In the books, the living Aegon Targaryen may, in fact, take offense when he finally finds out. Good thing the one on the show was killed in the cradle.