Чудовище means ‘a monster’. A beast is not always a monster.
Beast (from wiktionary)
1. Any animal other than a human; usually only applied to land vertebrates, especially large or dangerous four-footed ones.
— живо́тное, зверь.
2. (more specific) A domestic animal, especially a bovine farm animal.
— живо́тное, скоти́на.
3. A person who behaves in a violent, antisocial or uncivilized manner.
— живо́тное, зверь, скоти́на, тварь, чудо́вище.
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