Monsters of the River Nations is currently slated to have twenty-five new monsters either native to this stretch of land or not uncommon for the denizens here to associate with. At low levels the book focuses on semi-aquatic animals and plants like the hydrus that enters its victims through their mouth and makes its own exit out and the grappling vine that grabs pulls anything that moves towards its thorny mouth. Around the mid-levels, the book focuses more on playful fey like the half-halfling and half rabbit leshy or tormented undead like the autumn death.
Because the river nations adventure path focuses mostly on plains, forests, fresh-water aquatic, and a little on mountains, the book focuses on monsters native to these environments. There are a few extraplanars present but these types remain few and far inbetween. For players, Monsters of the River Nations holds a new playable race. The armouchi are a race of primatives, but that does not mean they cannot find a home among the civilized folk. Their origin is not entirely known. Some say they are the result of all the humanoid and monstrous humanoid races interbreeding. Others say the gods made them from whatever was left over after making all the "better" races. Human, elf, hobgoblin, and orc look down on this race equally. When they hit, however, you know it. But when you have a +4 Str, you don’t really care much about what the other races say.