Were the new monsters in Dragon Warrior Monsters already in Japanese Dragon Quest games?
Yes, but not all. Some of the slimes (SlimeNite and SpotSlime), Gasgon, probably some other monsters I’m not sure about, and probably about any bosses never seen in the US (Pizzaro, Mirudrass, Mudou, DeathMore, DarkDrium [Dark Dream]). Dragon Quest 6 had various colors of SpotSlimes, a MetalyNite, and a purple KingSlime.