@Princess_Fluttershy
Brightglow is from G1. A common misconception is that My Little Pony Tales was G2, but G2 didn’t have any cartoons. Also, the alicorn in that group is Dazzleglow, not Brightglow. Brightglow is the orange one.

@TwiNet  
We know nothing about Tia/Luna, but Lauren Faust wanted Tia mysterious and dificult to understand.  
But they are real Alicorns.  
Celestia Said in the Show: “the birth of an Alicorn is something Equestria has never seen”.  
I think you mean this, but Tia and Luna have Founded Equestria after the Events with the three Tribes, who we now know as the Hearts Warmin Eve Story.  
So it is clear, Equestria has never see an Alicorn Birth, because at the Time of Tias or later Lunas Birth, there was no Equestria.  
If I remember right, then in S9E17 (The Summer Setback) is the 1111 Year of Celestia on the Throne, so Equestria is somewhere between 900 and 1100 Years old.  
The word Alicorn was not used before S3E5 (Magic Duel) with the Amulet Name.  
The first Pony who was called Alicorn was Twilight, by Rarity in S3E13 (Magical Mystery Cure).  
Before Alicorns was just called Unicorns or winged Unicorns. Crysalis are counted as Alicorn Type too, even if they usual call her a Pegacorn.  
I have forget two other Alicorns: Brightglow, an G2 winged Unicorn and an winged Unicorn in the Hearts and Hoof Day Book, the CMC have in S2E17 (Hearts and Hooves Day).

@Princess_Fluttershy  
Flurry Heart’s the only natural born alicorn, as we know from Celestia and Luna. Celestia, Luna, Cadance, and Twilight all did something worthy of princessdom, so they became them, or were ascended by another alicorn. Sunny and Sunset Shimmer are what I like to call Half-Alicorns, where they emit spectral wings and horns in times of crisis or revelation, see the My Past Is Not Today video/Daydream Shimmer for Sunset in this case.

@Modish&ElegantPony  
Some say there are 2 Types of Alicorn, the natural born (Celestia, Luna, Flurry Heart) and the artifical created by Magic (Twilight, Cozy Glow, Pinkie Pie (Comic), Sunny Starscout).

So is this truly how Alicorns are born?

Good Catch.  
Oh, darn this would have gone nicely with my Foreshadowing collage.

Just the middle Window was missing, at the end there Was a Picture of the complete Window, but i not remember if there was a Earth Pony visible.  
Strange too, there was Twilights Symbol and a Poster of the Wonderbolts, but no Symbols of Celestia, Luna, Cadance or Flurry Heart.  
There was not even any other Creature then Ponys or Animals.  
They not even have mention the other Races.

Interesting observation! Like she was enchanted by the spirits of these figures chosen to represent the tribes.