Monsters (モンスター Monsutā or 魔物 Mamono, lit. "Mysterious Creature") are creatures that exist on the Dragon Quest universe.
In Dragon Quest main series titles, most monsters are enemies of the protagonist party. Beginning with Dragon Quest V, however, a monster recruitment feature was introduced, allowing the player to add certain monsters to the party in order that they may aid it throughout the story.
In the Dragon Quest Monsters series, monsters play a much larger role, as the player is required to recruit or scout enemy monsters to assemble a party. Once successfully recruited/scouted, and upon reaching certain levels, these monsters can be bred or synthesised by combining or evolving (depending on the mechanics) into stronger species with improved stats.
In some games, especially the spin off such as Dragon Quest Monsters series, Monsters who join the Hero party, are also known as Companion Monsters (仲間モンスター Nakama Monsuta, lit. Companion Monsters).