Gloria Anzaldua says that the shadow-beast is that part of women that scares men and makes them want to "control and devalue" women and their culture.
In a sense, then, the shadow-beast is the other. The other is that which is opposite of ourselves. As such, it is something that must always be controlled and devalued. Because it is our opposite, it is necessarily opposed to us.
Within our own selves, we try to suppress the parts of us that are (we think) opposite of what we want ourselves to be. In society, we do the same. We tend to see things that are unfamiliar and fear and suppress them rather than trying to broaden our horizons by understanding them.