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 in a deleted scene
Don was not part of his name; it was an honorific for a Spanish gentleman.
The character was featured in a deleted scene from "Shore Leave", wherein he accosted Yeoman Barrows, grabbing her from behind a tree, but was then fought off by Dr. McCoy and ran away. ([1]; Star Trek Magazine issue 163, p. 33)