Rotate gameobject accroding to ground slope ?!

Try

transform.rotation = Quaternion.LookRotation(transform.right, hit.normal);

RotateGameObjectAccordingToGroundSlope.cs

 using UnityEngine;
 public class RotateGameObjectAccordingToGroundSlope : MonoBehaviour
 {
     Vector3 _dir = Vector3.forward;
     Camera _camera = null;
     void Start ()
     {
         _dir = transform.forward;
         _camera = Camera.main;
     }
     void Update ()
     {
         Vector3 inputXZ = _camera.transform.right*Input.GetAxis("Horizontal") + _camera.transform.forward*Input.GetAxis("Vertical");
         inputXZ.y = 0;
         if( inputXZ.sqrMagnitude>0 ) _dir = inputXZ.normalized;
 
         var ray = _camera.ScreenPointToRay( Input.mousePosition );
         if( Physics.Raycast( ray , out var hit ) )
         {
             Vector3 tangent = Vector3.ProjectOnPlane( _dir , hit.normal );
             transform.rotation = Quaternion.LookRotation( tangent , hit.normal );
 
             transform.position = hit.point;
             Debug.DrawRay( hit.point , hit.normal , Color.yellow );
             Debug.DrawRay( hit.point , tangent , Color.cyan );
             Debug.DrawRay( hit.point , inputXZ , Color.magenta );
         }
         
         // var ray = new Ray( transform.position , Vector3.down );
         // if( Physics.Raycast( ray , out var hit , 1 ) )
         // {
         //     Vector3 tangent = Vector3.ProjectOnPlane( _dir , hit.normal );
         //     transform.rotation = Quaternion.LookRotation( tangent , hit.normal );
         // }
     }
 }

Here is a script that allows you to control this "horse" with a keyboard input. It requires NavMeshAgent component and a valid nav mesh.

tip: lower the Angular Speed on that NavMeshAgent component to something around 3.0. Default 120 is too much here.

> NavMeshAgentHorseController.cs

 using UnityEngine;
 using UnityEngine.AI;
 
 public class NavMeshAgentHorseController : MonoBehaviour
 {
     [SerializeField] NavMeshAgent _navComponent  = null;
     Vector3 _moveDirXZ, _moveNormal, _moveTangent;
     void Start ()
     {
         _moveDirXZ = transform.forward;
         _moveNormal = Vector3.up;
     }
     void Update ()
     {
         float h = Input.GetAxis("Horizontal");
         float v = Input.GetAxis("Vertical");
         
         _moveDirXZ = Quaternion.Euler( 0 , h * _navComponent.angularSpeed , 0 ) * _moveDirXZ;
         _navComponent.velocity = Vector3.MoveTowards(
             _navComponent.velocity ,
             _moveDirXZ * v * _navComponent.speed ,
             _navComponent.acceleration * Time.deltaTime
         );
     }
     void LateUpdate ()
     {
         if( Physics.Raycast( transform.position , Vector3.down , out var hit , 1f , _navComponent.areaMask ) )
         {
             Vector3 tangent = Vector3.ProjectOnPlane( _moveDirXZ , hit.normal ).normalized;
             _moveTangent = Vector3.Lerp( _moveTangent , tangent , Time.deltaTime*5f );
             _moveNormal = Vector3.Lerp( _moveNormal , hit.normal , Time.deltaTime*5f );
             transform.rotation = Quaternion.LookRotation( _moveTangent , _moveNormal );
 
             Debug.DrawRay( hit.point , hit.normal , Color.yellow );
             Debug.DrawRay( hit.point , _moveDirXZ , Color.red );
         }
     }
 }