A boot used in place of a shoe. Hoof boots are becoming more and more popular because of people learning about how bad horseshoes are. They are often used in the transition from horseshoes to barefoot, and theya re sometimes used for barefoot horses when walking on hard rocky ground, if the hooves have not been previously conditioned to the hard surface.
The best thing to do is carry an Easyboot or similar product. These are plastic and metal booties and keep the hoof protected. If fitted correctly, they are not painful and you can ride on them for quite a while. Of course, once you go home, be sure to call a farrier.
Dismount and go back. If you are trail riding, its always handy to carry an Easyboot; then you can put it on and continue riding. Make sure all nails, etc.. are out of the hoof. Save the shoe if you have it for the farrier. If you have a very tender-footed horse, and are on rocky terrain, you can try making a "wrap" around the hoof with a bandana and clothing or several layers of duct tape to minimize damage on the way back. Make sure its on tight.