The zombie's face animation is a nice touch, to make it clear to the player that something is happening in the game. The effect is quite subtle; due to the limited number of face bones that we can retarget in the Humanoid rig type, we are not able to get a full range of expression across in the zombie's face.
One way to get around this is to use a different kind of animation. Blendshape or Morph Target animation types do not use bones or joints to deform the model, but use a second, altered version of the mesh. Blendshape animation sequences can be driven in the animator the same way that skeletal animation can.
Blendshape animation sequences are usually unique to a model. They are created by making a copy of the whole or part of a mesh and then moving, scaling, and rotating vertices to change the shape—creating a facial expression or some other kind of deformation.
After the blendshape has been linked to the model, it can be exported...