怪兽 strange monster- like creatures dragon like creatures, for example. (often used in games or cartoon like thing) children use it a lot. If a grownup talks about this topic, normally they'll talk about their childhood thing or things of their kids or video games. 怪物: monster (for animals or human) adult might use this word a lot. He is a monster.

怪：奇怪---strange 兽：animals 物：things 怪兽--emphasize the animals monster , 怪物---for everything,including 怪兽

Totally agree with XiaoDeng. My opinion sometime you can mix it in informality.

不一样Monsters generally refer to strange animals, or terrible animals, and monsters can refer not only to animals, but also to people

不一样Monsters generally refer to strange animals, or terrible animals, and monsters can refer not only to animals, but also to people