*g* I'm pretty sure that it's the cracked-out x-files story, because I see it mentioned fairly frequently. It's called "A Fish Called Krycek." The real point here is that "mute" was meant to mean "almost totally incapable of communication with our hero." Mulder thinks of him throughout the story as "his pet" and says they don't know whether he's intelligent or a "dumb fish." Also, he masturbates to wildlife documentaries on mermen instead of porn. If anybody's read it more recently than I have, they could probably tell you whether there's any definitive sign of sentience at any point on the series (yes, series).

ALSO, I think that the laid-back kinda dorky mute mermaid story = teh awesome.