For this swap, Rainbowjuice listed two ponysonas, Barnacle and Salty, and Big Macintosh and Fluttershy. I went through quite a few concepts for each but ultimately decided on making the child for Big Mac and Fluttershy. This is the result:
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As you can see, she takes after her mother in coat color and talent(/cutie mark) and after her father in hair and eye color.
The concept of Murphy's Law applied greatly to this custom. Everything that could have gone wrong did go wrong, and before I knew it, it was too close to shipping time to fix... I hope Rainbowjuice really liked her...