The horse is grocery shopping. Is the horse supposed to pull the grocery cart? No. Clearly the horse has to push the grocery cart just like everybody else does. If everyone started pulling their grocery carts instead of pushing them, it would be chaos. CHAOS!
The cart has hay in it, and the horse is hungry. If the horse was before the cart, it would have to turn around to eat the hay, at which point the cart would be before the horse anyway, as it should have been to begin with. You want the horse before the cart? You want the horse to starve when there is a pile of perfectly good hay mere feet away, in the cart, all for the sake of your idiom?
The cart is actually a kart. You absolutely don’t want a go-kart racing up behind a horse. The horse is either going to kick the driver in the head, or the kart is going to crash into the horse at which point the driver gets crushed as the horse falls onto it. Either way, it’s bad news for the kart driver. If the kart is before the horse, the horse can make its own decision about whether it feels like racing the kart or going back and eating more hay from the cart—the hay you wanted to keep for yourself rather than letting the starving horse have some.
The cart is on fire. A burning cart attached to and behind a horse? What do you think the horse is going to do? It’s going to run, pulling the burning cart around wildly, setting fire to everything, not to mention shitting all over the place from the fear of being attached to a burning cart. The horse shit will also probably catch fire. What kind of chaos-monster are you? First you wanted to starve the horse, now you want to attach a burning cart behind it? At least if the burning cart is before the horse, the horse can see the fire and calmly walk away, or, if the fire isn’t too bad, roast some marshmallows, maybe make some s’mores. Even though you wouldn’t let the horse have any of your hay, it would probably share its s’mores with you, which is a good outcome for you and the horse.
The cart is being pulled by a different horse, or some other cart-pulling animal, maybe a reindeer, and our horse is riding a large skateboard and being towed behind the cart. This would be awesome, and both you and the horse would be Instagram stars—like break-the-Internet famous. Maybe it’s even a hoverboard rather than a skateboard, a la Back to the Future. Clearly the cart has to be before the horse riding the skateboard/hoverboard for this to work.
You want to have the cart before you commit to the horse, because if you don’t have the cart, and you get the horse, well…all you have at that point is an eating/pooping machine that you have to take care of. A cart, on the other hand, can be useful even if the horse never shows up. You can do lots of things with a cart, not so many with just a horse. Once you have the cart and are tired of pulling or pushing it around yourself, or trying to get the dog to pull it while you film it for Tik Tok, THEN you get the horse and you’re in business. You can deliver hay, you can pull other horses on skateboards. The sky’s the limit.