certain aspects or traits of a person or thing that reveal themselves, either occasionally or on a regular basis.
"It doesn't really surprise me that she said things to you. But don't take it personally. I mean, after all, she's generally a nice person, albeit a bit quirky at times. It's just the nature of the beast I guess."
The actual expression is hundreds of years old, and was made famous by Shakespeare in Othello.
BRABANTIO: What profane wretch art thou?
IAGO: I am one, sir, that comes to tell you, your daughter and the Moor are now making the beast with two backs.