Sometime after ‘punishing’ Chrysalis with her magical horse dicc (#2116621), like a good friend would, Twily wants to share those experiences with her friend so she message them to come over. For a science experiment.  
According to her plan, she needed at least one person to take all 6 cocks up their holes, multiple rounds without breaking, and non other than Pinkie has the mentality strong enough for that, and so the Pie Eating Contest begin…
 
Rainbow Dash arrives first, as she does in everything, and is overly excited to participate in Twily’s “experiment”  
“I know this is gonna be great, but you need to calm down” Twilight said while preparing the spell. She then tap on RD’s crotch, causing a large bulge which then instantly peel her pant open, reveal a dangling, erect human penis.  
“This is something…I did not see coming” - Dashies baffled at what just grew out from her body, and on the bed, Pinkie start stripping her clothes off. “Hey…wait…is this..what I think this is ?” - Dashies says with a nervous look on her face. “It’s gonna be a wonderful bonding experience” Twilight then push RD toward where Pinkie is, on her four, presenting her pie to RD. “Come on in and have some fuuuuun” Pinkie too is excited to see where this go.
 
Knock knock - AJ, Fluttershy and Rarity arrives, and as the door fly open, two of them audibly gasping at what they are seeing: Rainbow Dash humping Pinkie, like a rabbit having spasm. So then somehow Twily convinced them to participate in this little party, so Fluttershy get a dick, Rarity get a dick, Apple Jack- wait, AJ already have one - to everyone’s surprise. As RD finish pumping her seed into Pinkie, she lies down on the bed, exhausted, cum dripping off her flaccid meat “Best exercise ever !”
 
“Ready for the next one Pinkie ?” - AJ ask without hesitation, as she spreads her legs, presenting a massive erecting cock. It take only one glance at AJ’s meat for Pinkie to falls in love with it, and so she crawl down, onto AJ’s cock and start licking it while Fluttershy who is leaning against the wall, watching, completely naked, and her cock seem slightly aroused, which mean she probably ready for this. AJ then stir into Pinkie’s wet vagina, smearing those natural lubricant on her hand then grab on Fluttershy’s sausage, smearing love sauce all over it “Come over here and spread Pinkie’s butt for me will ya ?” - AJ inviting, while yanking on Fluttershy, pulling her over using her now full grown horse dick.
 
With her natural strength, AJ easily pick up thicc Pie and shove her cock straight up her friend’s canal, with both Pinkie’s squeal and the sound of AJ’s cock slamming into Pinkie’s cervix can be heard. With Pinkie now leaning against AJ’s body, her butt is in position for Fluttershy to insert her speciment. As it was lubricated by Pinkie’s pussy juice, despite it absurd size, Fluttershy’s cock still easily slide into Pinkie’s backdoor, they both then start their sexual motion, with rythmic moaning from all three.
 
Witnessing Pinkie’s canal being invade by AJ’s meat, so much so that the cum from previous session with RD start flowing out, get Rarity really excited, so had to help herself while waiting for her turn on Pinkie. No different on Twily, as she use Pinkie’s hand on her meat.  
Apparently Sunset did get the memo and walk in already stripped off of her clothings, regardless, the scene still baffles her as anyone would.