There are plenty of monsters out there that would like nothing more than to kill you, as the game features more than 150 different kinds for you to fight. Monsters have adaptable AI, meaning that they behave differently depending on the time of day and even in different weather conditions.
When a monster is defeated for the first time, they are listed in the in-game bestiary, along with their basic stats (level, hit points and faith). As the player kills more of that enemy, their bestiary entry is fleshed out to include more stats, such as resistances, their attack and defense, and even more.