A monster (aka mob) is an in-game, non-player entity that you, as a player, can fight.
Killing monsters is the major aspect of Atlantica PvE (player versus environment). Monsters come in groups, aka formations, and will attack you between 1 and up to 3 groups at a time, depending on your location.
Some monsters scale their level to the level of the highest Main Character in a Party when Combat begins. This can be good (Experience is based on the highest level character in the fight) and bad (Damage is based on the highest level character in the fight).
See List of Monsters