I want to rotate my chess pieces to -90° as they are spawning horizontally.

Questions & Answers
1

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

public class BoardManager : MonoBehaviour
{

private const float TILE_SIZE = 1.0f;
private const float TILE_OFFSET = 0.5F;

private int selectionX = -1;
private int selectionY = -1;

public List<GameObject> chessmanPrefabs;
private List<GameObject> activeChessman = new List<GameObject>();

private void Start()
{
    SpawnAllChessmans();
}

private void Update()
{
    UpdateSelection();
    DrawChessboard();
}

private void UpdateSelection()
{
    if (!Camera.main)
        return;
    RaycastHit hit;
    if(Physics.Raycast(Camera.main.ScreenPointToRay(Input.mousePosition), out hit, 25.0f,LayerMask.GetMask("ChessPlane")))
    {
        selectionX = (int)hit.point.x;
        selectionY = (int)hit.point.z;
    }
    else
    {
        selectionX = -1;
        selectionY = -1;
    }
}

private void SpawnChessman(int index, Vector3 position)
{
    GameObject go = Instantiate(chessmanPrefabs [index], position, Quaternion.identity) as GameObject;
    go.transform.SetParent(transform);
    activeChessman.Add(go);
}

private SpawnAllChessmans()
{
    activeChessman = new List<GameObject>();

    //Spawn White Team!

    //King
    SpawnChessman(0, GetTileCenter(3, 0));

    //Queen
    SpawnChessman(1, GetTileCenter(4, 0));

    //Rooks
    SpawnChessman(2, GetTileCenter(4, 0));
    SpawnChessman(2, GetTileCenter(7, 0));

    //Bishops
    SpawnChessman(3, GetTileCenter(2, 0));
    SpawnChessman(3, GetTileCenter(5, 0));

    //Kinghts
    SpawnChessman(4, GetTileCenter(1, 0));
    SpawnChessman(4, GetTileCenter(6, 0));

    //Pawns
    for (int i = 0; i < 8; i++)
        SpawnChessman(5, GetTileCenter(i, 1));
    
}

private Vector3 GetTileCenter(int x, int y)
{
    Vector3 origin = Vector3.zero;
    origin.x += (TILE_SIZE * x) + TILE_OFFSET;
    origin.y += (TILE_SIZE * y) + TILE_OFFSET;
    return origin;
}

private void DrawChessboard()
{
    Vector3 widthLine = Vector3.right * 8;
    Vector3 heightLine = Vector3.forward * 8;

    for (int i=0; i <= 8; i++)
    {
        Vector3 start = Vector3.forward * i;
        Debug.DrawLine(start, start + widthLine);
        for (int j=0; j <= 8; j++)
        {
            start = Vector3.right * j;
            Debug.DrawLine(start, start + heightLine);
        }
    }
    // Draw the selection
    if(selectionX >= 0 && selectionY >=0)
    {
        Debug.DrawLine(Vector3.forward * selectionY + Vector3.right * selectionX, Vector3.forward * (selectionY + 1) + Vector3.right * (selectionX + 1));

        Debug.DrawLine(Vector3.forward * (selectionY + 1) + Vector3.right * selectionX, Vector3.forward * selectionY + Vector3.right * (selectionX + 1));
    }
}

}

2

In the SpawnChessman function, when you instantiate the object, you can set its rotation. Replace Quaternion.identity with the rotation you need to make the prefab in the correct orientation.

 private void SpawnChessman(int index, Vector3 position)
 {
     GameObject go = Instantiate(chessmanPrefabs [index], position, Quaternion.identity) as GameObject;
     go.transform.SetParent(transform);
     activeChessman.Add(go);
 }
3

Can you please explain briefly