I read one once - brace yourself - it was Mulder/Krycek. Krycek was a mermaid, which in this universe was a fairly intelligent mammal - about dolphin level. They were also really rare, never been kept in captivity. Krycek is injured by a propellor, and taken to the aquarium where Mulder is a marine biologist. Scully is the manager. Mulder, in the course of his examination, collects semen from Krycek, and apparently in mermaid-world, a handjob means you're dating, because he becomes quite fixated on Mulder. Mulder thinks he's pretty, so one night he sneaks into the tank and he and Krycek hump. The next day Scully is like 'so there are cameras in the mermaid tank OH GOD YOU BANGED A FISH look take a holiday'. Mulder agrees he has clearly lost the plot, and that he'll take the next week off. Then he finds the camera blind spots and gets Krycek there so him can anally penetrate him, which Krycek seems rather dubious about. Then he goes on holiday, and Krycek pines. They release Krycek back into the wild at about the same time Mulder jerks off in the ocean, and Krycek tracks him down like a shark with blood. Anyway, I was expecting this whole fic to end with the reveal that mermaids are actually an intelligent species. Nope. Not at all. Mulder lives happily ever after dating an animal. He even thinks about how nice it is not to have to put up with insecurities and stuff. Also, Jim and Blair from Sentinel cameo as mermaids, and Mulder inspires them to try anal or something, IDK.

Aaaaaanyway, despite that I really love mermaid fics and have like five WIPs where Tony is a mermaid (why always Tony? DON'T KNOW) but it just occurred to me Wanda would make a fantastic mermaid.