Why is this such a rare pairing? Am I the only that sees an alternative version of Applebuck Season with Big Mac saying, "Y'know sis? I'll make a bet with ya. If you can harvest the entire orchard, I do your chores for a month. Wearin' a dress. You lose, you cut me a switch to spank ya with."

Because she totally deserved a roasted rump in that episode.

5865534 I don't think she deserved a spanking in that episode.

She deserves to get spanked for overestimating herself, while she vows to take on Big Mac's portion of the task because he's injured?

5865599 Yes. She put herself in danger when there were plenty of Ponies willing to help her.

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I have no opinion about the episode, so here's Equestria Girls Mac spanking Equestria Girls Applejack against his will:

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To be honest...I can imagine Big Mac giving her a bottom on fire as instructed by Granny Smith; on grounds that she earned one and Granny doesn't have the firm hoof she used to have

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Because their ages are close - they come off as peers, especially since the gap from them to Apple Bloom is a lot bigger. Throw in how they seem to act as substitute parents to her, and a dynamic where he has authority over her feels off.