I found this one out by accident today so I decided to throw a few more to confirm.
By using a Party G.R.E.N.A.D.E in a radius containing a hunter pet with a similiar model to that of a shaman or druid animal form you can make it dance. It's rather amusing especially when the hunter doesn't see the grendade go off.
This does not appear to work with non-combative pets. I've yet to try on non-pet mobs. All the times I've done it so far have been in stormwind in the trade district, I don't know if location matters, I don't think it would but I don't play a hunter so I'm limited to the pets I happen to see running around. It does not work on apes or other pets that can't be shape changed into, and it only seems to work on about 2/3 of cats....
One time I did it on a cat that looked like a cheetah and ... it kept sticking it's butt in the air instead of dancing... not sure what was up with that.
Screen Shots:
Ugly Dog
Ugly Dog Humping My Coyote Spirit
Frostsaber? ... not the Winterspring Frostsaber... but they might work too..
Spoted Somethingrather.
Have fun!
For those of you wondering you get the grenades from the "Slashdance" loot card... they sell on ebay for like 2-10 bucks and you get 99 of them per card.
Edit
One more screen shot I missed..