Japanese word of the day #21
He is a monster of a person
怪獣（かいじゅう） Is a word for monster that is the most commonly use. It doesn't always portray something violent and is more just like 'strange creature', though they can be considered to mean a dangerous creature
野獣（やじゅう）Is a word that is more along the lines of 'beast' and is then more of a violent term than 怪獣
モンスター Is just the word from English for 'monster'
There are a couple other words that are native to Japanese folklore that mean monster, but these are ones you'll see the most often