MouseRotateTransformY.cs

using UnityEngine;
using System.Collections;
 
public class MouseRotateTransformY : MonoBehaviour 
{
	private float sensitivity;
	private Vector3 mousePos;
	private Vector3 mouseOffset;
	private Vector3 rotation;
     
	private void Start ()
	{
		sensitivity = 0.4f;
		rotation = Vector3.zero;
	}
     
	private void OnMouseDown()
	{
		mousePos = Input.mousePosition;
	}

	private void OnMouseDrag()
	{
		mouseOffset = Input.mousePosition - mousePos;
		
		rotation.y = -(mouseOffset.x + mouseOffset.y) * sensitivity;
		
		transform.Rotate(rotation);
		
		mousePos = Input.mousePosition;
	}
}