> engineering literature from Russia from the 1960s, one sometimes finds references to a ‘water goat’

Oh, that's where it comes from!

My dad told me something close when I was a kid, but according to him it was the early EU doing dictionary translation between English and French and getting "water sheep". I never could find it.

Through an unrelated anecdote, in my head this is now bound to the music of "smoke on the water", with a sheep impression for the guitar (bass?) — "Sheep on the water, ba baa baaah, ba ba ba baaah"