[i dont play 40k]

As you all know, there are three forces of chaos: mortals, dameons and beasts. In one army, you can have all three of these forces mingled together. Mortals and daemons going together, I can understand completely as they're both direct servants of the chaos gods. Beastmen, on the other hand, only exist as a freak accident of what happened when the gate to heaven broke down and chaos mutated everything nearby. So why would these beastmen suddenly want to ally themselves with this mostly organised force?

[Stooga]

Most beastmen are worshippers of Chaos in one way or another, just like Marauders who often aren't direct servants under Chaos, but follows and worships it to some degree.