Dragon Quest Monsters (titled Dragon Warrior Monsters for the US release) is the first game in the Dragon Quest spin-off series, Dragon Quest Monsters. Released for the Nintendo Game Boy Color in 1999, it stars a young Terry from Dragon Quest VI as the hero.
Whereas most game in the series focus on the adventures of a hero rising up against the forces of darkness, DQM is the much smaller-scale story of a little boy and girl, Terry and Milly, characters from Dragon Quest VI, when they were just small children. It opens with Milly's kidnapping by an evil creature called Warubou, after which another called Watabou appears, explains the situation, and starts Terry off on his quest to rescue his sister.
DQM took the monster-recruitment elements of Dragon Quest V and expanded upon them, also borrowing a few mechanics from the Pokemon series, setting the standards for what would become the "Dragon Quest Monsters series." The basic concept is that Terry, as a young boy still too weak to fight for himself, must enlist the help of various creatures to act in his stead. Collected monsters level up and grow like a character in a main-series Dragon Quest game might. As he progresses, he is able to add more and stronger monsters to his collection, which stands in for usual Dragon Quest power escalation elements such as steadily strengthening spells.