Cleaned up the script so it now works independant of gravity. Added a second Mover script that controls the speed and direction the boulder moves once triggered. Added a target to the prefab as this is the point at which the boulder moves towards.

game/Assets/Prefabs/Traps/RollingBoulder.prefab

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game/Assets/Scenes/Main.unity

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game/Assets/Scripts/Traps/Mover.cs

@@ -0,0 +1,24 @@
+using System.Collections;
+using System.Collections.Generic;
+using UnityEngine;
+
+public class Mover : MonoBehaviour
+{
+    [SerializeField] private float speed;
+    [SerializeField] private GameObject target;
+
+    // Start is called before the first frame update
+    void Start()
+    {
+        
+    }
+
+    // Update is called once per frame
+    void FixedUpdate()
+    {
+        //transform.LookAt(player.transform);
+        Quaternion targetRotation = Quaternion.LookRotation(target.transform.position - transform.position);
+        transform.rotation = Quaternion.Slerp(transform.rotation, targetRotation, 1 * Time.deltaTime);
+        transform.position += transform.forward * speed * Time.deltaTime;        
+    }
+}

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game/Assets/Scripts/Traps/RollingBoulder.cs

@@ -5,12 +5,10 @@ using UnityEngine;
 public class RollingBoulder : MonoBehaviour
 {
     Rigidbody rb;
-    // Float for speed of the arrow.
-    [SerializeField] private float speed;
     // Float for time in seconds to wait to destroy the arrow on contact with any other gameobject that is no the player.
     [SerializeField] private float waitToDestroy;
-    // Vector3 to set direction of travel for the arrow once the trigger is activated.
+    
-    //[SerializeField] private Vector3 direction;
+    public GameObject target;
 
     private void Start()
     {
@@ -22,7 +20,9 @@ public class RollingBoulder : MonoBehaviour
         // Checks to make sure other collider is the Player using tag.
         if (other.gameObject.CompareTag("Player"))
         {
-            rb.useGravity = true;
+            Mover mover =
+            gameObject.GetComponent<Mover>();
+            mover.enabled = true;
         }
     }