Okay, we got a new one, boys.
My analysis of "Asimov's Tail:"
It's the inverse of Chekov's Gun ("If there's a gun on the table in act 1, it should go off in act 3" or someone like that). Asimov's Tail is like saying "If a gun goes off in act 3, it should be on the table in act 1."
You don't need that for guns because we all know what guns are. A character in a modern setting pulling out a gun to win a fight on page 120 isn't going to bother the reader. But a tail is unexpected: You need the reader to buy in to and expect the tail before it becomes plot-relevant, or it'll read like a deus ex machina.
(Another comparison is Chekov was talking about the stage, and Asimov is talking about books. Asimov's Tail may not be as applicable to the stage, where the audience can all clearly see the character's tail in his costume.)
Separately, Asimov doesn't just say "You have to describe the tail," but "Someone has to step on the tail." Your character's trait or ability that will help them in a critical moment later in the plot must also have downsides. This will make the character feel more real, even if they're fantastical and alien.
tbh, i think "it must have downsides to make the character feel more real" really undersells what this accomplishes.
You set up the tool of their victory as a personal *challenge* they must overcome. This makes the victory more surprising, character-building, and satisfying. The very trait, tool, or skill that was presented as a weakness becomes a strength.
