With Discovery bringing back the Talosians and Lower Decks bringing back the Pakleds, it occurred to me that Star Trek sometimes goes quite a long time without any mention of a particular species, only for them to reappear decades later. I'm curious as to what the longest gap in appearance is for any Star Trek species in canon sources.
Being mentioned or unambiguously alluded to counts as an appearance; the species does not need to appear "in the flesh." The gap is measured in real-world days between the release/broadcast of the films/episodes, not in-universe time between the events. Behind-the-scenes material such as cut footage, script iterations, DVD special features, or other production material that does not appear on screen in canon does not count.
For example, the Talosians appeared in TOS: The Menagerie, Part 2, originally broadcast November 24th, 1966, and did not appear in Star Trek again until this little guy:
(who counts as an unambiguous allusion) appears in VOY: Future's End, Part 2; 13 November 1996; 10,947 days (29.99 years) later.
The Pakled gap is not as large as I expected, since they had background appearances on DS9 I wasn't aware of. Based on Memory Alpha's list of appearances, their gap is from DS9: Let He Who Is Without Sin... (11 Nov 1996) to LD: No Small Parts (8 Oct 2020), giving 8,732 days (23.92 years).
The references to the Pakleds and Talosians in the shooting script of Star Trek: Nemesis do not count as none of these ideas made it into the final cut.
So the Talosians are the current reigning champs. Who, if anyone, can beat them?
Edit to clarify, I consider all officially licensed film and television productions to be canon, including Lower Decks and Short Treks. No novels, video games, or other media, and no un-licensed works.