Hey, how's everybody doing? I noticed someone doing this on my server today and they told me how (so, credit to Mylegsgone - Blade's Edge US). Sorry if this is a repost, please don't flame if it is but I looked and didn't find it. Also, feel free to move the post if it's in the wrong section.
Steps:
1. Buy some http://www.wowhead.com/item=58489/potion-of-illusion (Potion of Illusion).
2. Hotkey your 2-person mount. After testing, I have had success with this on Obsidian Nightwing and Sandstone Drake, so it's either 2-person mounts or transformation mounts that work. I can't be sure.
3. Right click your Potion of Illusion, and at the same time, begin to mount with a hotkey. If done correctly, you should transform about halfway through the cast bar of your mount, and when it succeeds, you should be a white version of whatever the potion turned you into, with your mount on your shoulder/arm.
This is server-sided, meaning that everyone can see it. When someone right-clicks you to mount you, they will look just like your mount - being carried on your shoulder. When you accomplish this, the Potion of Illusion effect does disappear, so this lasts as long as you are mounted. Dismounting cancels the effect, as does relogging, but it persists through /reload.
Screenie: imgur: the simple image sharer
UPDATE! This also works with the mage glyph-spell, Glyph of Illusion (http://www.wowhead.com/spell=63092/glyph-of-illusion), which does the same thing as Potion of Illusion. It also also works with Blingtron's spell, Let's Party! Although, when done with Blingtron, it produces a different effect. See new screenshot.
Blingtron Screenshot: http://imgur.com/DCRcUtD