Mojang has pushed a new snapshot early (or late since last week's never came) this week, with a few very interesting little additions. Firstly, "Mobs can now ride stuff that rides other stuff." That explains the screenshot of a skeleton riding a pig riding a spider that we say a few days ago. Also new is the ability to keep riding a minecart/pig/etc even after you log off and log back on again. Finally some new language additions were added for those of you outside the English-speaking world, and a few bug fixes to top it all off. You can quest your thirst for recreating the cat on the horse photo at the source link below. Yee-haw!
Source: Mojang