Not everyone is a Vulcan, a Klingon, or even a Borg, and we can't all be as ruthless as a Romulan, as acerbic as an Andorian, or as financially savvy as a Ferengi. Whilst Star Trek has given the world the longest of lists of unforgettable alien species, known far beyond the fanbase, it's not unsurprising that, in its nearly 60-year history and nigh on 900 episodes, there are those that might have slipped your mind entirely.
The very first episode of Star Trek ever aired — The Man Trap — was 'monster-of-the-week' type fare, the 'Salt Vampire'/M-113 creature only making a (living) re-appearance in Star Trek: Lower Decks (and technically in the Short Trek Ephraim and Dot). By the very nature of episodic television, this one-and-done approach was fairly common, even, at first, for the show's most memorable 'monsters' like the Gorn.
Most of the species here have appeared in only one or two episodes, never to be seen, spoken of, or referenced again — as if flushed out of Cargo Bay 101 whilst Tom and Seán twirl their moustaches in the control room. Hopefully, this list will serve to jog your memory of those aliens assigned to oblivion. We've got a Lethean — you've perhaps forgotten about them too (?) — and a multitronic engrammatic interpreter on standby just in case.
Get ready for lots of '-ans' and '-ians'!
10. The Axanar
You're certainly already familiar with the name 'Axanar'. It's the title of the much-discussed fan film project led by Alec Peters that tells the story of Garth of Izar and crew during the 'Four Years War' with the Klingons, ending in Garth's ultimate victory at the titular planet. Prelude to Axanar, released in 2014, features a gaggle of famously familiar faces and Star Trek alum the likes of Tony Todd, Gary Graham, J.G. Hertzler, and BSG's Kate Vernon and Richard Hatch.
In all this, you've perhaps forgotten that we've actually met the Axanar. They were one of the first (friendly) species the crew of the NX-01 made contact with, thanks in large part to the linguistic genius of Hoshi Sato. The Axanar are androgynous with 400-plus-year lifespans, so Captain Garth could even have fought alongside Hoshi's first contact friend at the Battle of Axanar.
The adversaries in that skirmish have never been given in canon, nor have we encountered the Axanar again since Fight or Flight.