I can literally see the story to this:

Rainbow Dash: "Hey AJ! Have you maybe still some cider from this year left over?"

AJ: "Sure, Rainbow Dash. But you don't get it for free... You have to play my baby for one day!"

Rainbow Dash: "..."

AJ: *giggled* "Oh, relax Dash. That was just a..."

Rainbow Dash: "Ok, deal!" *flys away*

AJ: "joke... Wait, what? Rainbow?"

*Rainbow comes back... dressed in a Diaper and walked with a crinkling butt over to AJ and hugs the stunned Farmpony*

Rainbow Dash: "I hope you will take good care of me... mommy."

AJ: "... äh... sure... Sugarcube." *pats the back of her now little foal for the day*

AJ makes the best Mommy, well ok 2nd best to Fluttershy would but still she's just as good in her own way. ^_^

and dashy makes the best foal, just a good combination