Spherical legs rolling where that enemy go and not looking

Sorry if its a stupid question...

In my game there is an enemy with a ball instead of legs. The enemy can look at me and go to different directions. How I can make the ball rolls allways to the direction he go and not the direction he look and roll accordingly to the speed of the enemy?

The ball attached to the enemy and don't have any code / rigidbody.

Maybe something like this can get you started?

function Update () {
if (Input.GetKey ("up")) {
rigidbody.AddForce (Vector3.-down * 10);
}
if (Input.GetKey ("down")) {
rigidbody.AddForce (Vector3.down * 10);
}
if (Input.GetKey ("left")) {
rigidbody.AddForce (Vector3.-right * 10);
}
if (Input.GetKey ("right")) {
rigidbody.AddForce (Vector3.right * 10);
}
} 

Ofcourse then you'll need to add a rigidbody to it.