Detecting moment when GameObiect is created in hierarchy.

As iabulko suggested, use hierarchWindowChange and keep track of the number of objects.

 using UnityEngine;
 using UnityEditor;
 
 [InitializeOnLoad]
 public class SuperHelpfulEditorClass {
 
     public delegate void HierarchyChangeAction(GameObject go);
     public static event HierarchyChangeAction onGameObjectCreated;
 
     static int objectCount = 0;
 
     static SuperHelpfulEditorClass()
     {
         EditorApplication.hierarchyWindowChanged += OnHierarchyWindowChanged;
     }
         
     static void OnHierarchyWindowChanged () {
 
         GameObject[] objects = (GameObject[]) Resources.FindObjectsOfTypeAll (typeof (GameObject));
 
         if (objects.Length > objectCount)
         {
             GameObject go = Selection.activeGameObject;
 
             if (onGameObjectCreated != null)
                 onGameObjectCreated(go);
         }
 
         objectCount = objects.Length;
     }
 
 }

New GameObjects are selected by default, not sure how reliable this is, so it might have to change. The event can be used like this:

 using UnityEngine;
 using UnityEditor;
 
 [InitializeOnLoad]
 public class ShowCreatedGameObjectName {
 
     static ShowCreatedGameObjectName()
     {
         SuperHelpfulEditorClass.onGameObjectCreated += OnGameObjectCreated;
     }
 
     static void OnGameObjectCreated(GameObject go)
     {
         Debug.Log("GameObject was created.\nGameObject.name = " + go.name);
     }
 }