The border traverses a variety of terrains, ranging from major urban areas to uninhabitable deserts. Approximately 350 million legal crossings occur annually, and is the most frequently crossed border in the world. The total length of the continental border is 1,954 miles (3,145 km).
Only a veterinarian will know what's wrong with your dog but usually stumbling, wobbling and falling down is associated with neurological problems, especially if there aren't any other signs. I would really take him to the vet as soon as you can.
Some horses stumble because of a joint problem. For example, the knee joint might suddenly give and buckle forward, and unless the horse can catch himself with his other leg, he could go down in front. This might happen at any gait, making the horse dangerous to ride.