How do monsters live? It takes not only the expected nutrients (varied based on species), but also a particular type of energy (name pending). This energy is absorbed passively, and only found in dungeons or specially-made barns.
How do monsters sustain themselves outside dungeons or a barn? Wild monsters do not, but captive monsters often have a collar, which can be purchased either at the farm supply store or the guild.
How expensive are the collars? Collar prices vary based on the needs of the monster, including size and MAG stat. The higher the MAG stat, the more expensive a collar is needed to ensure the monster is kept healthy.
Can monsters speak? Not using any human language, but intelligent monsters can speak the monster languages. The reason for this is that humans are blessed by one deity (information pending) and monsters are blessed by another (information pending). Monsters and humans can come to understand each other, but only to a minor degree, and cannot learn each other's languages.
Are monsters intelligent? Most humanoid monsters are intelligent, but most humans in-setting don't realize that it can be on-par to human intelligence. The exceptions to this are dungeoneers who at times must deal with the more intelligent monsters.