The answer to all you questions is "No". Not many native speakers know the word 'behemoth'. Those who do use it it like this.
behemoth /bɪˈhiːmɒθ/ n
- a gigantic beast, probably a hippopotamus, described in Job 40:15
- a huge or monstrous person or thing
hippopotamus /ˌhɪpəˈpɒtəməs/ n ( pl -muses, -mi / -ˌmaɪ/)
- a very large massive gregarious artiodactyl mammal, Hippopotamus amphibius, living in or around the rivers of tropical Africa: family Hippopotamidae. It has short legs and a thick skin sparsely covered with hair
Here is the picture of the animal.
This is my question: Can we say that Behemoth is the synonym of Hippopotamus? Suppose that I go to a jungle and see a hippopotamus. Then I come back home and say to my father: "Daddy, I saw a Behemoth"
Will my father be sure that my meaning of (Behemoth) was (Hippopotamus) or it can refer to other big animals too?
I mean the picture I have uploaded.
Many thanks in advance.