word choice - Difference between 獣｛しし｝ and 野獣｛やじゅう｝ (beast)

I learnt the words 獣{しし} and 野獣{やじゅう} and their definitions look too similar. Can someone explain me the difference between them?

While searching more for beast words, I also found that 獣 could also be spelt けもの or けだもの (also written in katakana　ケダモノ).

Answer

獣 is always read けもの or けだもの in modern Japanese.

けもの

Simply refers to any kind of beast or animal.

けだもの

It's only used for emotionally deprived, unscrupulous, monstrous individuals, like a murderer, rapist or barbarian. It always refers to the actual perpetrator, so for example, you wouldn't call Adolf Hitler a けだもの. Think of Chucky from Child's Play.

野獣{やじゅう}

Also means beast or brute, but it doesn't carry any negative connotation like けだもの.

しし is not used in modern Japanese as 獣, but 獅子{しし} is an outdated form of ライオン.