The information is good but the final part is not correct or at least inaccurate. You do not need to speak to the Horse Guai in Chapter 5 and you do not need to finish the whole Chapter to obtain the Dark Thunder Transformation. All you need to do is to win the boss ♛battle against the Ying Yang Fish in the palace. After that fight (in which you also have to fight the horse guai in a🔥 sort of spirit form) you can go to the location of the cart and you will directly have the option to pull forcefully and will obtain the transformation.