Beastmen start with Horns and with Mutation and Strength access you they can make great players. Read this for suggested builds and uses.
Descriptions on the individual Chaos Player positions, the roles they can fill and skills you should select for them.
Chaos can hire a Minotaur Big Guy player to add even more strength to their team. Coupled with some mutations the Minotaur can be a great addition.
Chaos Warriors are more agile than most ST4 players, though also provide some more muscle to the Chaos team. Read this for suggested builds.