The song is from season 2 episode 18 of MLP:FiM.
♫ Lyrics for the song:
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[Pinkie Pie]

He had a Cranky Doodle sweetheart
She's his cranky doodle joy
I helped the Cranky Doodle boy, yeah!
I helped the Cranky Doodle boy!

Cranky Doodle Donkey and Matilda: Pinkie!
Pinkie Pie: Whoops, privacy. Sorry.