Our 3 year-old quarter horse has what looks like a hard lump under her jaw. There are also some lump

A complete oral examination is definitely needed here as soon as possible. Possible causes are an impacted tooth, an abscessed tooth root, trauma to the jaw, cancer as well as some other less common conditions. At 3 years of age, she'll be in the middle of shedding 24 deciduous teeth or "caps". It is common to develop lumps under the jaw and in the maxilla bone (upper jaw) during this period. They are usually not a problem, unless they become more enlarged than the other side, hot, painful, or start discharging pus. In racehorses the lumps in the upper jaw can be so large to inhibit airflow through the nostrils — which reduces their ability to breathe properly during exercise. Sometimes the cap or the permanent tooth erupting from the bone below the cap may become "jammed", and so pressure builds up near the root. This is a serious condition, because the pressure in the hard lump may get so great that it bursts to the outside skin, and bacteria enter the tract and set up a tooth root abscess. This is very serious in young horses, and often means they need to have the permanent tooth extracted. This is both expensive and a major procedure which seriously impacts on the lifetime health and function of the mouth. Thus prompt treatment to "release the jammed tooth" is indicated ASAP. An x-ray is often needed to differentiate exactly what the hard lump is, and how it is best treated.