From source:

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Yeeeessss I’m a sucker for Appledash so I had to add it to my headcanon.

In my canon they started dating right around the beginning of season one and dated on and off until like the end of season three. They were always competitive with one another and one night they got in a heated argument about whether or not the other had cheated in their hoof-wrestling match. They were both a little drunk and Rainbow, ever turned on by Applejack’s competitiveness, leaned in and Applejack, ever turned on by Rainbow’s persistence, obliged. They hooked up and after that the chemistry only grew, they ended up in a sort of casual relationship for a while but they broke up and got back together frequently over little arguments. In all truthfulness they were both a bit too stubborn to communicate and listen to each other's feelings and it’s eventually why they decided to break up. They easily slipped back into the friendship they had had pre-relationship and had a much better relationship as friends than they had as girlfriends. They still have sexual tension from time to time though lol