a narwhal is "el unicornio de mar"

While German is the hands-down winner for having a single word for something that English does not have a single word for (Weltschmertz!), I kind of like that Spanish has a word for "the only thing" (el único), as the use of "thing" in English adds imprecision where it need not exist.

"Lo único que se debe hacer" is more concise than "The only thing you have to do." Why do we need a thing in there? Ideally, "thing" should be reserved for unidentified objects.

The use of "thing" in this case is, I think, an imperfection in English similar to the necessary use of an antedecentless pronoun in "It's raining." We can't say "The children enjoy lollipops, but it's not happening today," because there's no singular noun that could be the "it." But in "It's raining," the "it" just can't be helped. That's a lack of intelligent design.

Incidentally, the Spanish word for "unicorn" is "el unicornio," which sounds a little bit dirty.