Eustace was originally popularized by St. Eustace, who was born a Norman nobleman and is said to have been converted to Christianity by seeing a crucifix between the antlers of the deer he was hunting. It was introduced to England by the Normans, and can be found in medieval legend. In literature the name appears in the person of Eustace Clarence Scrubb in C. S. Lewis's The Chronicles of Narnia.
The image of Eustace today is as sedate and stuffy as the monocled Eustace Tilly character on The New Yorker magazine covers. Its diminutive Stacy became a unisex hit.