Just like people, animals adapt to their physical limitations extremely well. Although having a portion of the tongue missing may decrease his efficiency in eating or grazing, the horse may still be able to adapt and become a useful, enjoyable companion and mount.
Some questions to consider would be:
1. How much of the tongue is actually missing?
2. Is the tongue long enough so the horse's mouth can properly accommodate a bit for riding (assuming that riding is a purpose of the horse)?
3. Can the horse graze normally and eat grain without problem?
We recommend that you consult a veterinarian to help determine the answer to these questions and to identify any other issues or concerns which should be considered.
