kisame, I suppose you're right. Instead it should attack both kingdoms in the war, even things up a bit. (the percentages will be bigger for the bigger kingdom, help stop people unfairly picking on small kingdoms)
Good points also maia, killing it should yield an equally appropriate reward. 50% could be divided up among the kingdoms depending on where it spent the most time, while the other 50% could be divided depending on who put the most effort into killing it. If we go with multiple types of monster idea, then it could change the rewards too, magical monsters give magical bonuses, crashed ufo's give tech bonuses, etc. However, I think it should be the other way around, King Kong would be beaten by Godzilla after all, but I think multiple types of monsters might just be taking things a bit too silly.
I like the thief op leading the monster to a target idea too. Then a kingdom could choose to either play cooperatively and share in the bonuses for winning, or just be conniving and backstab the other island members. Combined with the attack bonus for working with another kingdom it would make for some cunning strategy, especially by the new lizardman race.