Sometimes a film is so good that it transcends its genre. The Host is one of the most acclaimed foreign films of the last decade and its beast deserves much of the praise. As Godzilla was Japan's cinematic allegory for the nuclear attack from the United States, The Host uses a fictional beast to incite satire for government occupation.
A monster can metaphorically represent any number of things, but if it doesn't scare the hell out of its victims, it's worthless. The monster in The Host is so wild and unpredictable that it turns the film into a horror movie.
It's more the presence of a hidden beast that creates fear and tension than its destructive behavior. The real question is whether its size is an issue; it's big enough to destroy bridges, so it is a relative giant.