Leyara's Locket is so far the only item that I've found to allow mounting up without removing other transformation buffs, also going to Brunnhildar village can be used if you've done quests there but that's limited to Northrend.
Edit: If you don't have the locket or are low level, you could also do it by getting the curse from Cursed Sailor, HS, mount up, ask someone to decurse or wait for 2min.
1. Use the transformation that you want to mount up with, for example drink Noggenfogger Elixir until you've turned into skeleton
2. Use Leyara's locket
3. Mount up
4. Cancel the locket buff, and you will stay mounted as a skeleton.
Same works with many other non-mountable transformations too, although for most you can use only flying mounts. Doesn't seem to work with potion of illusion, nor with wasp from underbelly elixir. Also I've heard rumors that in 4.3 you can't mount up with any transformation anymore(false, it still works), so have fun while it lasts.
Here's pics for Gordok Ogre Suit, Noggenfogger Elixir, Rituals of the New Moon, Iron Boot Flask, Underbelly Elixir and Shabby Arakkoa Disguise
![]()