Yeah, that must be the reason. I'm not familiar with mediaeval myths about unicorns, so it means pretty much “a horse with a horn” (but I wouldn't count an oryx as one -- the uni- part means it has to only have one horn, doesn't it :-)), but on the other hand I know about the myth of the mermaids' singing (and Ulysses's strategy to cope with it) so I wouldn't count the top half of a woman glued onto the bottom half of a fish as one.