Whether it's a gift horse or not, you should look in your horse's mouth!
Have you noticed your geriatric horse having trouble taking treats such as apples/carrots, resisting the bit, headshaking, or becoming more fussy about having his/her mouth handled? Equine odontoclastic tooth resorption and hypercementosis (EOTRH) maybe be responsible for some of these behaviors.
This disease of the incisors and canines (front teeth) is most common in horses over 15 years old. Some evidence of this disease can be noticed by your veterinarian during an oral exam: gingival(gum) recession allows more space for feed to become trapped between the incisors, cause infection, and weaken the periodontal ligament and alveolar bone surrounding the root that hold the tooth in place (tooth resorption). In later stages of this disease cementum (the calcified part of the tooth covering the root) proliferates (the hypercementosis part of the name) in an attempt to stabilize the tooth, so a "bulbous" appearance of the tooth may be noted, as shown in the picture.
The only cure for this disease is extraction of the teeth. Your veterinarian will take radiographs (x-rays) to determine how much of the root remains to determine the best approach to extraction. The radiograph below shows lysis (black holes) of the corner incisors where there has been severe tooth resorption.
In the early stages, pain control and management of the gingival (gum) infection by rinsing the mouth with dilute chlorhexidine solution and administering antibiotics if necessary can help slow down progression of the disease.
Many horse owners are pleasantly surprised with the positive personality change in their horse once the incisors are extracted and the source of chronic pain removed. While they initially need to be on a pelleted feed, most horses without incisors transition well to eating grass and hay, because they use the molars (cheek teeth) to chew food.