public string ReturnAniName(int num){
     int i=0;
      foreach(AnimationState state in animation)
       {
           if(i==num)    
            {
              return (state.name);
            }
           i++;
       }
     return null;
}

        [MethodImpl(MethodImplOptions.InternalCall), WrapperlessIcall]
         internal extern AnimationState GetStateAtIndex(int index);

using UnityEngine;

public static class AnimationExtension
{
     public static AnimationState GetStateAtIdx(this Animation animation, int num)
     {
         if(animation = null) return null;
         int i = 0;
         foreach (AnimationState state in animation)
         {
             if (i == num) return state;
             i++;
         }
         return null;  
     }
}

AnimationState state = animation.GetStateAtIdx(i);