I summoned a pig riding an XP orb with this command:

/summon Pig ~ ~ ~ {Saddle:1,Riding:{id:"XPOrb"}}

because it would allow me to do this: http://i.imgur.com/6JE8am3.gif

I noticed that if you pick up the XP orb, for a moment you are unable to kill the pig using melee weapons. I also tested with cows, same result.