How is it possible mc?

I know the frsge sounds a bit strange but how can I make a horse on a minecart in minecraft? So the horse should then hang on a minecraft on a leash and also follow minecraft

Not possible, the Minecart is too fast for the horse and there's not even the possibility

You can put some animals and mobs, generally NPCs, in Minecarts by spawning them in the area or nearby and then go in by yourself, but I don't know if it works with horses.