• first you need declare array like this:

public List<GameObject> PrefabsArray;

• second, some method to instantiate prefabs and then delete it. wait a little and instantiate again, like this:

IEnumerator RandomInstantiator()
{
    while(true)
    {
        GameObject instance = Instantiate(PrefabsArray[UnityEngine.Random.Range(0, PrefabsArray.Count)]) as GameObject;
        yield return new WaitForSeconds(5f);
        Destroy(instance);
        yield return new WaitForSeconds(1f);
    }
}

• third, you need to start this 'instantiator' like this

void Start()
{
    StartCoroutine(RandomInstantiator());
}

• fourth, just attach script to any gameobject in scene, attach prefabs and you got it 8)

note: C#

finally

/* CODE C# */

using UnityEngine;
using System.Collections;
using System.Collections.Generic;

public class PowerUps : MonoBehaviour
{
	public List powers;
	private float time = 5f; // in seconds

	void Start()
	{
		StartCoroutine(RandomInstantiator());
	}

	IEnumerator RandomInstantiator()
	{
		while (true)
		{
			float addXPos = Random.Range(-4, 4);
			Vector3 spawnPos = transform.position + new Vector3(addXPos, 3, 0);
			Instantiate(powers[randomPowerUps()], spawnPos, Quaternion.identity);
			yield return new WaitForSeconds(time);
		}
	}

	int randomPowerUps()
	{
		return UnityEngine.Random.Range(0, powers.Count);
	}
}

Thanks Scroodge =) although it had settled, I leave how to fix it .

/* CODE C# */

using UnityEngine; using System.Collections;

public class PowerUps: MonoBehaviour {

private ArrayList powers;
private float time = 300;
private float count;
public GameObject prefSmoke;
public GameObject prefBomba;
public GameObject prefCondor;
public GameObject prefGas;
public GameObject prefIce;
public GameObject prefOtherBall;
public GameObject prefOvni;
public GameObject prefPiranha;
public GameObject prefSmokeBlack;
private GameObject randomPowerUp;

void Start()
{
    powers = new ArrayList();
    powers.Add(prefSmoke);
    powers.Add(prefBomba);
    powers.Add(prefCondor);
    powers.Add(prefGas);
    powers.Add(prefIce);
    powers.Add(prefOvni);
    powers.Add(prefPiranha);
    powers.Add(prefSmokeBlack);
}
void Update()
{

    count++;
    if (count == time)
    {
        float addXPos = Random.Range(-4, 4);
        Vector3 spawnPos = transform.position + new Vector3(addXPos, 3, 0);
        Instantiate(powers[randomPowerUps()] as GameObject, spawnPos, Quaternion.identity);
        count = 0;
    }
}

int randomPowerUps()
{
    int rand = Random.Range(0, powers.Count - 1);
    return rand;
}

}