Random spawning in Unity (C#)

To keep track of what objects you instantiated you could create a List of GameObject and then when you instantiate an object you can add it in the List. Then you can use any GameObject in that list to do whatever you need.

public GameObject[] arrows;
public float interval = 5;
private List<GameObject> myObjects = new List<GameObject>();

// Use this for initialization
void Start()
{
    StartCoroutine(SpawnArrow());
}
public IEnumerator SpawnArrow()
{
    WaitForSeconds delay = new WaitForSeconds(interval);
    while (true)
    {
        GameObject prefab = arrows[UnityEngine.Random.Range(0, arrows.Length)];
        GameObject clone = Instantiate(prefab, new Vector3(0.02F, 2.18F, -1), Quaternion.identity);

        //Add the object to the list
        myObjects.Add(clone);

        yield return delay;


    }
}

And for spawning object like that it would be better to use InvokeRepeating, you can pass it the repeat rate and when you want it to start. More information on InvokeRepeating

This is how it could look like with InvokeRepeating

public GameObject[] arrows;
public float interval = 5;
private List<GameObject> myObjects = new List<GameObject>();

// Use this for initialization
void Start()
{
    InvokeRepeating("SpawnArrow", 0f, interval);
}

void SpawnArrow()
{
    //Check if there's something in the list
    if (myObjects.Count > 0)
    {
        //Destroy the last object in the list.
        Destroy(myObjects.Last());

        //Remove it from the list.
        myObjects.RemoveAt(myObjects.Count - 1);
    }

    GameObject prefab = arrows[UnityEngine.Random.Range(0, arrows.Length)];
    GameObject clone = Instantiate(prefab, new Vector3(0.02F, 2.18F, -1), Quaternion.identity);

    //Add the object to the list
    myObjects.Add(clone);

}

You can destroy the last spawned object in the list by doing Destroy(myObjects.Last()); it will return the last object in your list and destroy it. You should then also remove it from the list myObjects.RemoveAt(myObjects.Count - 1);