From the White Wolf Wiki, the most complete White Wolf reference

The Beast refers to the innate demonic predator that awakens in each and every vampire upon their Embrace. The Beast stands in direct opposition to a vampire’s Humanity (and in some cases the Paths of Enlightenment) and is responsible for many of the debased urges Cainites feel on a nightly basis. In times of extreme duress the Beast can overwhelm a vampire and force hir into a state of pure animalistic fight or flight, which is referred to as Frenzy or Rötschreck.