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Fourteen years ago, he had huge numbers at his command — not just witches and wizards, but all manner of Dark creatures.
~ Sirius Black talks about the evil creatures that Lord Voldemort commanded during the First Wizarding War.

Monster Masters are villains (whether they are human-like, monstrous, or supernatural) who might or can command creatures of different sizes or powers, such as Kaiju or packs of rats, but they are linked by having command over all kinds of monsters and creatures to do their bidding. 

A Monster Master is different than a Summoner in a state of constant command. That is to say, if someone calls forth monsters from a different place or dimension, he or she is a summoner; if he or she has an army of monsters with him or her, they are a Monster Master.