Hey, I've noticed there was no answer for this question so I thought I'd give it a shot! I'm looking for the same thing... I'm almost done working on a script to do this but I need to figure out how to lock the rotation. So far, if you apply this script to an enemy, and add the "Player" tag to your player, than the enemy will follow the player but when it gets too close, it will rotate showing the other sides of the enemy (a 2D sprite... so it's not pretty lol) but for the most part, this one works.

pragma strict

private var Xpos : float; private var Ypos : float; private var max : boolean;

var Vert : boolean; var maxAmount : int; var step : float;

function Start () { Xpos = transform.position.x; Ypos = transform.position.y; }

function FixedUpdate () {

if (Vert) { if (transform.position.y >= Ypos + maxAmount) { max = true; }

     else if (transform.position.y <= Ypos)
     {
         max = false;
     }
 }
 
 else
 {
     if (transform.position.x >= Xpos + maxAmount)
     {
         max = true;
     }
     
     else if (transform.position.x <= Xpos)
     {
         max = false;
     }
 }
 
 if (Vert)
 {
     if (!max)
     {
         transform.position.y += step;
     }
     
     else
     {
         transform.position.y -= step;
     }
 }
 
 else
 {
     if (!max)
     {
         transform.position.x += step;
     }
     
     else
     {
         transform.position.x -= step;
     }
 }

}