is it? because i've heard reference to aegon in the tv show as the promised prince (which is why rhaegar kept naming his sons that in the book), and it's probably why they held off revealing the name until the finale. aegon's a fixated name for rhaegar for a reason, and i'm pretty sure the name was actually mentioned in the series, just not in relation to jon.

so by that logic, rhaegar passed that hope along to his wife, which is why without rhaegar there to enforce it, she still named him aegon targaryen to her brother on her deathbed. she could just as easily have given him a stark first name, but for all we know, rhaegar convinced her their baby was some special coming, which is why she insisted ned protect him.