If you look carefully at the front part of Applejack’s cart that fell off here, you’ll notice something interesting: the part was held on by “mortice and tenon” joints. The little tabs on the front part are the tenons, and they fit in the slots on the main part, the mortices.
 
Mortice and tenon construction is often used for plastic kid’s toys, where the parts “snap into place”, which is kind of appropriate here. But we also get a bonus reference: that kind of construction is very old-fashioned, used in the days when nails were scarce and expensive. It makes sense that a traditionalist pony like Applejack would still be building carts that way.