Don Juan was a robot constructed by the planet computer of the Shore Leave Planet. The robot was a representation of the legendary Spanish seducer of females, Don Juan.
In 2267, he appeared to Yeoman Tonia Barrows on the planet during shore leave. The computer was able to probe the minds of the people on the planet and recreate their thoughts. Barrows wanted to be romanced and had chosen the famous lover as her beau. (TOS: "Shore Leave")
Don Juan was played by James Gruzal, who was uncredited for the role.
Don was not part of his name; it was an honorific for a Spanish gentleman.