Before u try this, u will need the and ( if u wanna ride as a fulborg).
Ok here is the trick, get some noggerfogers from Tanaris and use it till u turn yourself into a skeleton or just use your Dartol's rod of transformation, then activate your orb of deception that will turn u into the opposite faction. Now just get on your mount and right click the Orb of deception buff ( or change trinket). And there u go, now u can ride as a fulborg or a skelie