Just messing around on my spriest, searched the forums, and didn't find anything. Feel free to shame me down if this is a repost.
If you use Shadowform + Spectral Gryphon - Spell - World of Warcraft (Obtained via SoR), your mount will be almost completely transparent. It will look like you're riding on nothing.
To achieve a greater transparency, you can use Ethereal Oil - Item - World of Warcraft.
Screenshots:
http://i49.tinypic.com/kd39t1.jpg
http://i50.tinypic.com/5fmal0.jpg