Beastmen start with Horns and with Mutation and Strength access you they can make great players. Read this for suggested builds and uses.
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.