You will receive a free Neutral monster in your monster pen when you reach a specific number of monsters, whether you scouted or synthesized them. As long as you owned the monster, it will count:
MONSTER - HOW TO UNLOCK Drakularge - have owned 100 different monsters Grandpa Slime - have owned 200 different monsters Great Argon Lizard - have owned 50 different monsters Metal King Slime - have owned 150 different monsters
In Dragon Quest VI you must activate Dreamsharing, while in Dragon Quest IX you must activate Tag Mode.
MONSTER - GAME Mottle Slime - Tag Dragon Quest VI Teeny Sanguini - Tag Dragon Quest IX Malevolamp - Tag Dragon Quest VI Noble Gasbagon - Tag Dragon Quest VI Overkilling Machine - Tag Dragon Quest VI Shogum - Tag Dragon Quest IX Slime Stack - Tag Dragon Quest IX