Obsoleto, ver "AnimatorStringListAttributeDrawer.cs" >> https://gist.github.com/431b16af9a8d8da5f92d8aec163ccf2a.git

AnimatorVariableAttributeDrawer.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
#if UNITY_EDITOR
using UnityEditor;
using an = UnityEditor.Animations;
#endif

public class AnimatorVariableAttribute : PropertyAttribute { }

#if UNITY_EDITOR
[CustomPropertyDrawer(typeof(AnimatorVariableAttribute))]
public class AnimatorVariableAttributeDrawer : PropertyDrawer
{
    readonly static GUIContent[] noMulti = new GUIContent[] { new GUIContent("Multiobject editing no permitido") };
    readonly static GUIContent[] noAnimator = new GUIContent[] { new GUIContent("Animator no hallado") };
    readonly static GUIContent[] noAnimCont = new GUIContent[] { new GUIContent("Animator sin Animator Controller") };
    readonly static GUIContent[] noBehav = new GUIContent[] { new GUIContent("No reconocido como behaviour") };
    readonly static GUIContent[] noGameObj = new GUIContent[] { new GUIContent("No asociado a GameObject") };

    public override void OnGUI(Rect position, SerializedProperty property, GUIContent label)
    {
        if (property.propertyType == SerializedPropertyType.String)
        {
            position.width -= EditorGUIUtility.singleLineHeight;
            EditorGUI.PropertyField(position, property);
            position.x += position.width;
            position.width = EditorGUIUtility.singleLineHeight;
            if (property.serializedObject.isEditingMultipleObjects)
            {
                EditorGUI.Popup(position, -1, noMulti);
            }
            else if (property.serializedObject.targetObject)
            {
                Behaviour beh = property.serializedObject.targetObject as Behaviour;
                if (!beh)
                {
                    EditorGUI.Popup(position, -1, noBehav);
                }
                else if (beh.gameObject)
                {
                    Animator animator = beh.GetComponent<Animator>();
                    if (animator)
                    {
                        string[] anims = AnimatorListaDeVariables.ListaDeVariables(animator);
                        if (anims == null) EditorGUI.Popup(position, -1, noAnimCont);
                        else
                        {
                            int sel = 0;
                            for (sel = 0; sel < anims.Length; sel++) { if (anims[sel].Equals(property.stringValue)) break; }
                            EditorGUI.BeginChangeCheck();
                            sel = EditorGUI.Popup(position, sel, anims);
                            if (EditorGUI.EndChangeCheck())
                            {
                                property.stringValue = anims[sel];
                                property.serializedObject.ApplyModifiedProperties();
                            }
                        }
                    }
                    else EditorGUI.Popup(position, -1, noAnimator);
                }
                else EditorGUI.Popup(position, -1, noGameObj);
            }
        }
        else
        {
            EditorGUI.LabelField(position, label, "AnimatorStateString solo con strings");
        }
    }
}

public static class AnimatorListaDeVariables
{
    public static string[] ListaDeVariables(Animator anim)
    {
        if (anim)
        {
            if (anim.runtimeAnimatorController)
            {
                an.AnimatorController cont = anim.runtimeAnimatorController as an.AnimatorController;
                if (cont == null) return null;
                else
                {
                    string[] salida = new string[cont.parameters.Length];
                    for (int i = 0; i < salida.Length; i++)
                    {
                        salida[i] = cont.parameters[i].name;
                    }
                    return salida;
                }
            }
            else return null;
        }
        else return null;
    }
}
#endif

ExampleAnimVarisAttDrw.cs

/*
Add this to a GameObject with an animation controller
*/
using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class ExampleAnimStatStrAttDrw : MonoBehaviour
{
    [AnimatorVariable]
    public string ejemplo = "";
}