A replica of the legendary seducer produced by the machinery of the shore leave planet in 2267 from Yeoman Tonia Barrows's passing daydream, which attacked her and tore her uniform.
Don Juan appears in the episode Shore Leave. On an apparently uninhabited world in the Omicron Delta region, whatever the Enterprise landing party thinks of is quietly manufactured beneath the surface and sent up to meet them.
Yeoman Barrows, walking alone, lets her mind wander to the sort of romantic encounter such a setting calls for. A caped figure in period costume promptly appears and assaults her, ripping her tunic before running off as Kirk and McCoy arrive in answer to her screams.
She identifies him, somewhat shamefaced, as Don Juan. He reappears briefly later, and the incident helps the party see the link between their thoughts and the apparitions. He was played by James Gruzaf.