Before Star Trek: The Motion Picture, Spock (Leonard Nimoy) was going to be replaced by a Vulcan named Xon (David Gautreaux), but the character never made it to the big or small screen. Xon was initially created for Star Trek: Phase II, Gene Roddenberry's sequel to Star Trek in the 1970s. Star Trek: Phase II was to be the flagship series of Paramount's planned TV network which failed to materialize. In turn, Star Trek: Phase II was ultimately scrapped in favor of turning Star Trek into a feature film in the wake of Star Wars' blockbuster success.
Lieutenant Xon was created to take Spock's place as Science Officer aboard Star Trek: Phase II's new U.S.S. Enterprise. In the mid-1970s, Leonard Nimoy distanced himself from Star Trek, partly due to his not wanting to be typecast as Spock, but also because he was in a legal battle with Paramount over royalties from the studio merchandising his likeness as the Vulcan. After Star Trek, Nimoy had gone on to star in the Mission: Impossible TV series and the actor also wasn't interested in returning to a grueling weekly TV schedule. Therefore, for Star Trek: Phase II, the Spock character was figuratively split into two new creations: Xon, a full-blooded Vulcan who would be the new Science Officer, and Commander Will Decker (Stephen Collins), who would serve as Captain James T. Kirk's (William Shatner) new First Officer.
When Paramount decided to turn Star Trek: Phase II into a feature film, Xon was initially part of the movie's plans. This changed when Robert Wise (The Day the Earth Stood Still) was hired as the director of the Star Trek film. Although Wise wasn't a fan of the TV series, his wife Millicent loved Star Trek and she was outraged when she learned that Spock wasn't going to be in the movie. Convinced by his wife about how vital the Vulcan was to Star Trek, Wise returned to Paramount and demanded that Leonard Nimoy reprise his role as the Vulcan. Studio heads Michael Eisner and Jeffrey Katzenberg, who agreed that fans would be furious if Spock was left out of the film, caved and immediately opened negotiations with Nimoy to resolve their legal dispute, which happened within a week. Satisfied with the outcome, Nimoy agreed to look at the script for what would become Star Trek: The Motion Picture and he eventually signed on to bring Spock to the big screen.
But the addition of Leonard Nimoy's Spock to Star Trek: The Motion Picture was bad news for Xon. After all, since the original Vulcan was now in the movie, there was no need for his replacement. A revised draft of the screenplay saw Xon as the victim of a lethal transporter malfunction, but David Gautreaux did not want his Vulcan to be introduced only to be immediately killed off. Instead, the Vulcan who dies in the transporter accident at the start of the film was a new character, Commander Sonak (Rashad Beyzade Kamal). Xon was written out of the film entirely but, as a consolation, Gautreaux appeared at the start of Star Trek: The Motion Picture as a human Starfleet character named Commander Branch.
While Will Decker and Ilia (Persis Khambatta), the other characters created for Star Trek: Phase II, appeared in Star Trek: The Motion Picture (and were killed off), Xon never canonically appeared in any Star Trek movie or TV series. However, Xon was finally mentioned in the Star Trek: Lower Decks episode "Crisis Point"; the Vulcan was the butt of a joke where Ensign Beckett Mariner (Tawny Newsome) told Ensign Brad Boimler (Jack Quaid) that he would "probably be a Xon" and get cut out of the holodeck movie she created. When Boimler asked, "What's a Xon?" Mariner replied, "Exactly!"
The idea of a Vulcan who would replace Spock aboard the Enterprise was revisited twice, with Lt. Saavik (Kirstie Alley) in Star Trek II: The Wrath of Khan and Lt. Valeris (Kim Cattrall) in Star Trek VI: The Undiscovered Country. However, both female Vulcans were proteges of Spock who would have had his blessing to succeed him. Instead, Xon — the Vulcan who never was — simply became a piece of obscure Star Trek trivia.