"The nature of the beast" is a mildly jocular phrase. It means "its/his/her character".
Here is an example:
A: "MrP has just replied to one of my posts. Unfortunately his reply is incomprehensible. He doesn't seem to realise that if my English was good enough to understand his reply, I wouldn't have needed to ask the question."
B: "That's the nature of the beast, I'm afraid."
— i.e. it is characteristic of MrP to provide incomprehensible replies.
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