This bust is a copy of a famous Roman sculpture known as the Rondanini Medusa. The portrait was widely admired, because it depicted the features of the Gorgon in a more human form than normal. The hair, for example, is not a mass of snakes; only two serpents protrude from the hair, entwining at the neck in anything but a terrifying manner.
Nollekens, an Englishman, created this bust as a luxurious souvenir for well-to-do tourists. He and the 9th Earl of Exeter met in Rome (where the sculptor lived from 1760-1770) in 1764, when the Earl was travelling in the company of Lord and Lady Spencer, the actor Garrick, and Lord Palmerston.