I was thinking of monsters in opposing pairs, and concluding that to pitch up against a unicorn, the obvious thing would be a poisonous snake.
After which I thought, and how obvious is that? How well known is it that unicorns can purify water?
The one critiquer who didn't realize that unicorns had an association with virgins -- when a character thought only virgins were safe in a forest with a unicorn -- was probably an anomoly, but -- yes, the target audience for a story featuring a unicorn is probably mostly people who've read a lot of stories with unicorns. But it's never wise to unnecessarily limit your audience if the necessary knowledge can be larded in here and there. . .
Purifying water is probably in need of a clue. Virginity I'm not so sure of. Too much exposition, and you won't reach all those who don't know, and you drive off those who do, who get bored.