However, not every alien race in the “Star Trek” universe has left a lasting impact. While some have become iconic, others have faded into obscurity. Many of these lesser-known races were initially introduced with great promise but later vanished without explanation, leaving fans puzzled. Though a few have made sporadic returns, their prolonged absence remains a mystery.

The original 1960s “Star Trek” series introduced several alien races that have since become staples of the franchise. The Klingons, Romulans, and Vulcans continue to feature prominently in modern “Star Trek” iterations. Yet, other races, like the blue-skinned, antennaed Andorians, who debuted in the episode “Journey to Babel,” have not maintained the same level of presence. Described as fierce yet integral members of the United Federation of Planets, the Andorians initially seemed poised for significant roles.

The Andorians made a notable comeback in “Star Trek: Enterprise,” the 2001 prequel series starring Scott Bakula. Updated with a more intimidating appearance compared to their original ’60s depiction, they became a central element of the show, gaining prominence through the character of Admiral Shran, portrayed by Jeffrey Combs.

As one of humans’ earliest allies, you’d expect the Andorians to play a major role in the “Star Trek” films. You might even think they’d be featured regularly on spin-off series in the ’80s and ’90s. But other than a few background cameos and appearances in the 1974 animated series, the Andorians disappeared from screens for decades. They were absent from all three ’90s “Star Trek” shows, “The Next Generation,” “Deep Space Nine,” and “Voyager,” and there is never any indication that they aren’t still major players in galactic affairs.

Of course, the Andorians would return with a vengeance in “Star Trek: Enterprise,” the 2001 prequel series starring Scott Bakula. The Andorians are given a visual overhaul, looking far more menacing than their sillier ’60s counterparts, and became a major part of the series. They even received their first iconic hero character, Admiral Shran, played by Jeffrey Combs.

The classic 1966 episode “Journey to Babel” helped expand “Star Trek” lore, introducing not just the Andorians but also the Tellarites. They are said to be another of the first founding members of the United Federation of Planets, and are known for their argumentative nature. Visually, they are depicted as pig-like, with a prominent snout. After a couple of appearances, they, like the Andorians, were nowhere to be seen for decades, appearing in none of the ’90s spin-offs.

Thanks to their status as founding members of the Federation, though, the Tellarites did return in “Star Trek: Enterprise” alongside the Andorians. Because the series was set in the years just before the Federation’s founding, both the Andorians and Tellarites returned to play major roles in the series. But the Tellarites’ appearance offers up a solid explanation for their disappearance from screens: Even more than the Andorians, the Tellarites receive a major visual overhaul, thanks largely due to more advanced makeup FX available in the 21st century. Ditching the cheap latex mask, the Tellarites get a major upgrade, complete with boar-like tusks, which allowed the actors underneath to express themselves through their makeup. Suddenly, what was once a race limited to small roles could now expand, and “Enterprise” gave us several memorable Tellarite characters as a result, appearing in some of the show’s best episodes.

In 1973, “Star Trek” went boldly to Saturday mornings, revived for a brief, 22-episode run of animated episodes. Now referred to as “Star Trek: The Animated Series,” the cartoon revival took full advantage of its hand-drawn nature and featured several new, wholly inhuman alien races. This included the Enterprise’s new bridge officer, M’Ress (Majel Barrett), a feline-like humanoid alien from the planet Cait. The Caitians, however, following a lone background appearance in “Star Trek IV: The Voyage Home,” weren’t seen again until 2022.

The irony, of course, is that the Caitians’ return came in the first animated “Star Trek” series since 1974, the adult comedy “Star Trek: Lower Decks.” There, we meet another Caitian crew member, Dr. T’Ana (Gillian Vigman), the resident Chief Medical Officer of the USS Cerritos. It’s probably not that ironic, though, if we’re being honest, because — like the Tellarites — it’s likely that the reason for the Caitians’ absence had something to do with the potential cost of a full-body furry costume.

When “Star Trek: The Next Generation” premiered in 1987, there was a clear and apparent effort to separate themselves from the classic 1960s series. Outside the Klingon Lt. Worf, there were few returning alien races, at least right away. This could partly be why the Andorians and Tellarites were absent, but it’s also likely the reason why those early seasons introduced so many new recurring aliens to canon, including the Pakleds, from the episode “Samaritan Snare.”

In that 1988 episode, the Pakleds are dim-witted aliens who struggle to fly their own ships. They trick the Enterprise into a trap, but it’s one that Captain Picard (Patrick Stewart) and stalwart crew quickly outmaneuver because the Pakleds are simply not smart enough to avoid being outwitted. They return a couple of more times, but after that, the Pakleds are not seen again until 2022, when they return for “Star Trek: Lower Decks.”

The Pakleds probably weren’t featured after their initial appearances because they were more than just dumb, they were one of the worst “Star Trek” villains ever and only good for a bad laugh. So it’s probably not surprising that their comeback came on “Lower Decks,” where comedy is the goal.

The Romulans were revealed as twins of the Vulcans in their dramatic debut, “Balance of Terror.” But that wasn’t the end of their shocking revelations, because in 2002’s “Star Trek: Nemesis” — the fourth and final “Star Trek” film with the cast of “The Next Generation” — we discover that they have a subspecies that evolved on a sister planet called Remus. These Remans were kept as forced laborers for centuries, and evolved in the mines, giving them a vampiric, nearly Nosferatu-like appearance.

In the film, the Remans revolt against the Romulan Empire, led by Shinzon (Tom Hardy), a twisted clone of Jean-Luc Picard. The end of that film sees the Romulan Empire and the United Federation of Planets holding historic peace talks, and while Shinzon and his Reman uprising were defeated, it seemed to signal a new era of cooperation between the three races. But this major new faction of aliens only appeared once more, in a two-part storyline in “Star Trek: Enterprise” that aired a few years after “Nemesis” was released. Of course, those two episodes, “United” and “The Aenar,” took place more than a century before “Nemesis,” so it didn’t continue their story.

Since that time, the Remans have never again appeared on-screen. “Star Trek: Lower Decks” did make a joke about them, but even when “Star Trek: Picard” dealt with the destruction of Romulus, we never got a glimpse of what happened to the Remans.