A year or two ago I read a fic where Cas was some kind of sea creature - like a merman, but tentacled, as I recall. So was Gabriel. And they ended up mated with Sam and Dean.

Totally AU - the boys aren't hunters. I've searched my links, and the tags here, and no luck so far.

Does this ring any bells for anyone? *hopes*

FOUND! Despite my misremembering the Sam?Gabriel aspect (it's just Cas/Dean). Many thanks to arashi89 for linking me to Specimen 001