The Beast is the second thing that makes Satsuki a formidable enemy. The first would be her intellect. The Beast was most likely created stemming from the incedent that occured in Satsuki's side story. In her side story Satsuki is being forced to pretty much "perform" her intellect for the Freemasons, in a test. During a break in the testing, Satsuki talks with the computer she's hooked up to and says that she had grown tired of the place they are in. The computer agrees with her then responds with the word "Kill" repeating on a single screen. And Satsuki decides that they should leave.
However it is uncertain as to how The Beast really did come along. And it all is really just speculation until something is mentioned in the manga.
The Beast is often called by Kanoe, a "pet" or Satsuki's "pet", as though it may be truely alive. Which The Beast really does consider itself to be. Alive that is. And it does appear to harbor the ever so human feeling of jealously when Yuuto spends a little too much time with its beloved master.
The Beast does something similar to what Yuzuriha's Inuki does; provide a protection. The Beast protects Satsuki by becoming her weapon and being able to control electronic what nots from the sanctity of the Government Building. It can be similar to how Inuki is able to transform into weapons and shields to protect Yuzuriha. However The Beast may be a little overprotective of Satsuki, seeing the way it seems to dislike Yuuto. It will be interesting to see how CLAMP decides to end this all.