I was debating who to ship Twilight Sparkle with for awhile, mostly changing my NG based on Twilight Ships I saw other people have and not really actually thinking about the ships I like.
 
Now Rainbow/Pinkie was one of my OG ships and I forgot about it for years, that is until I started seeing some RainbowPie art from various users recently which reignited my love for the ship.
 
Okay now that we’re past my own feelings for the ship now onto an actual explanation of the relationship:  
Twilight Sparkle developed a crush on Pinkie Pie not long after she (Twilight Sparkle) had moved to Ponyville but being the awkward nerd she is, was slow when it came to admitting her feelings. During this time Rainbow Dash and Pinkie Pie went from being affectionate best friends to girlfriends, and Twilight under the assumption that her romantic feelings were one sided decided to put her energy into Friendship lessons/reading/organizing/typical Twilight stuff.
 
Then Twilight became a Alicorn Princess and the friendship lessons were replaced with Twilight teaching her new students.
 
At Big Macintosh and Sugar Belle’s Twilight had a little too much cider and “accidentally” told Pinkie how she felt about her.
 
Unbeknownst to Twilight, Pinkie had romantic feelings for Rainbow and Twilight, and after Twilight’s confession, Pinkie was the one to officially start the poly relationship. Originally Twilight and Rainbow were under the impression that they only had feelings for Pinkie, but later the two developed romantic feelings for each other as well.
 
When Celestia and Luna retired, the mane six got their powers, but outside of Applejack the rest of mane six instead of ascending to alicorn hood they changed in other ways.
 
Pinkie Pie became a Pegasus Dracony*, Rarity a Kirin, Fluttershy a Bat Pony, Rainbow didn’t change species wise (making her the odd one out) but her wings did become twice the size of a normal pegasi’s wings.
 
A few months after the six were offically coronated as the new leaders of Equestria, Twilight/Pinkie/Rainbow got married and had their first foal Moonlight Twinkle, with Pinkie being the one who gave birth and Twilight being the sire.
 
Moonlight’s dull colors comes from both Pinkie’s side of the family and her coat markings come from Twilight Sparkle’s mom who I headcanon as a Dapple Grey like several other Canterlot Unicorns.
 
When Moonlight was 7, Rainbow Dash gave birth to the twins Nightglow and Confetti, Twilight and Pinkie were the sires this time.
 
Pinkie Pie was turned into a Stallion in order for them to have Nightglow*.
 
Pinkie Pie is Mom, Twilight is Mama, and Rainbow is Mum, relationship wise the three are one of the outwardly affectionate and cuddly of the parents, outside of Capper and Rarity of course.
 
Pinkie Pie is the main instigator of said cuddle times, she’s always been a affectionate and a very emotionally tuned pony, and it’s no different when it comes to her wives.
 
*My headcanon when it comes to MLP ponies having foals is that while cis same sex couples can indeed have children (via alicorn magic) said children are always the same gender as the parent. While with same sex couples where one of them is trans/one uses a gender changing spell it’s the typical sperm + egg = baby routine i.e baby’s gender isn’t determined by the parent.
 
Outside of that Pregnancy works basically the same as a human pregnancy just with 11 months instead of 9.
 
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