noun
monster; giant monster
Typically refers to a large, destructive, often fictional creature, especially in kaiju films and tokusatsu. Can also be used figuratively for something huge and threatening.
ゴジラは有名な怪獣です。
Godzilla is a famous monster.
怪獣映画が好きです。
I like monster movies.
怪物 is a broader term for any monster or monstrous being, not necessarily giant or destructive. 怪獣 specifically evokes a large, often city-smashing creature.
モンスター is a loanword used for monsters in general, including in games and fantasy. 怪獣 is more strongly associated with Japanese tokusatsu and giant creatures.
Compound of 怪 (kai, 'mysterious, strange') and 獣 (jū, 'beast, animal'). The word was popularized in the mid-20th century through kaiju films.