1660s, native to South America and perhaps so called either because it was first brought back to Britain aboard Guinea-men, ships that plied the triangle trade between England, Guinea, and South America [Barnhart, Klein]; or from confusion of Guinea (q.v.) with the South American region of Guyana (but OED is against this). In the extended sense of "one subjected to an experiment" it is first recorded 1920, because they were commonly used in vivisection experiments.