Galen (Doctor)

From Memory Alpha, the free Star Trek reference.

Doctor Galen was an inspirational medical practitioner whose name is among several Starfleet doctors, their patients, and medical personnel in historical and literary contexts, which are included in the Emergency Medical Hologram programming.

When asked by Kes of the name individual included in that programming who inspired The Doctor, which he could name himself after. Among the individuals chosen were Dr. Galen, Dr. Salk and Dr. Spock. None of those names, however, were ones The Doctor could see himself with. (VOY: "Ex Post Facto")

"Galen" is likely a reference to the famous ancient Greek physician of the same name.