My horse has a lump under its jaw with no other symptoms of strangles. What else could it be?

The lump could be caused by a number of things. Even though there are no other symptoms of strangles, you may not be able to rule that out. Other possible conditions could be an abscessed tooth or a swollen lymph node that may be caused by some other health condition.
We cannot diagnose these conditions online. We recommend that you contact your veterinarian for a complete assessment. You can provide your veterinarian with good information such as the horse’s vital signs (temperature, pulse, respiration, gum color, hydration status, etc.) and details about the “lump” (how long it has been there, exact location, whether there is heat in it, etc.) to give the veterinarian the best opportunity for a correct diagnosis.