Peaceful monsters are those that do not attack you. They can be attacked, but only with a penalty to your alignment. An attacked peaceful monster also becomes hostile: "The dwarf gets angry!". This is acceptable for late game characters, but dangerous for low level characters with low alignment. Pets do not become hostile even if accidentally or purposefully attacked by the player. Pets can and will attack peaceful monsters with no penalty to the player, other than the chance of the pet being killed.