How to rotate an object parallel to the y-axis that is parallel to the x-axis?

Well this is kind of confusing -> How to rotate an object parallel to the y-axis so that it is parallel to the x-axis? , and the code that shows, "but" maybe it's me, but looking at your code and this other part -> How can I make them parallel to the x axis? I will leave you some code so that you can determine or understand what they do, I hope it is what you are asking.

If I understand your question correctly, you want to instantiate it on the x-axis, but you are passing it as a position vector, the position of the object that the script has, in this way -> transform.position, I imagine that because of the name of the class, maybe it is a dummy object that you will have for the scene, even though it does not have any associated "mesh" model, it has a transform, hence you can do -> transform.position, well this works but it is taking the position in space where you have the object that the script has, so it will be instantiated at that position:

    Instantiate (obj [Random.Range (0,obj.Length)], transform.position,
                 Quaternion.identity);

Now try to use Vector3.rightwhat will instantiate it in position x + 1relative to space, as I imagine that the movement for this object will be controlled by the script of the object in question that is the one you are instantiating or from somewhere else.

    Instantiate (obj [Random.Range (0,obj.Length)], Vector3.right,
                Quaternion.identity);

Still if the instantiation doesn't have the rotation you expect based on the original model, you can use what Arie CwHat says about using Quaternion.LookRotation(Vector3.up)try that in addition to the above Vector3.right.

    Instantiate (obj [Random.Range (0,obj.Length)], Vector3.right,
                 Quaternion.LookRotation(Vector3.up));

You say ... I have already put in the generator the objects that have to be generated in the correct way, parallel to the x-axis... - where you have put it.

Suppose you have something like this:

    //esto puede ser una simulacion de lo que usted dice que ha puesto.
    Vector3 pos = new Vector3(10, 0, 0);

So we use it like this:

    Instantiate (obj [Random.Range (0,obj.Length)], pos,
                 Quaternion.LookRotation(Vector3.up));

Well, as it was said before transform.position, you are passing the position of the object that the script has, but if you only want to take the position of the object that the script has, but only in xthis one, you can try something like this:

    Vector3 solo_x = Vector3.zero; 
    solo_x.x = transform.position.x;

    Instantiate (obj [Random.Range (0,obj.Length)], solo_x, 
                 Quaternion.LookRotation(Vector3.up));

    InvokeRepeating ("Generar", 3, 180);

Note the change of transform,positionfor others Vector3.

On the other hand if what you want is to control the rotation with which the object is instantiated you can try something like this:

    Vector3 solo_x = Vector3.zero; 
    solo_x.x = transform.position.x;

    //esta es la rotacion en la que queremos que se instancie, puede modificarla y ajustarla
    //asiganmos la rotacion que queremos atravez de un Vector3
    Vector3 rotar_y = new Vector3 (0, 90, 0);

    Instantiate (obj [Random.Range (0,obj.Length)], solo_x, Quaternion.Euler(rotar_y.x, rotar_y.y, rotar_y.z));
    InvokeRepeating ("Generar", 3, 180);

In the above we use Vector3 rotar_yto assign the rotation based on Euler ¹

But it Instantiaterequires a Quaternionfor rotation so we can use Quaternion.Eulerit like this,

Quaternion.Euler(rotar_y.x, rotar_y.y, rotar_y.z);

to get a Quaternionwhich is what we use in the method Instantiateseen in the code above:

¹

Euler angles constitute a set of three angular coordinates that serve to specify the orientation of a reference system of orthogonal axes, normally mobile, with respect to another reference system of normally fixed orthogonal axes.