No I'd have to disagree there, because the guy at the farm explains that there is one (The text makes it reasonably clear). I think what has happened here is a mistake with the writing (or deliberate mistake so that the player gets to fight it). The scene of what Sephiroth has done to it is really a flaw in the game flow, since you see the MS there swimming about and then instantly it is dead. The best way to really think about it is in the story, Sep kills it, and in the game, it is alive purely as a gameplay element.
If a story was made of VII it would be easy. Cloud is told about the monster, but when he reaches the swamp he sees that it has already been killed. (Obviously in the game this would be boring).
Also... the way they did the killing is plain stupid. How did Sephiroth get it spiked on a pole? And was what the pole doing there? haha