Making mobs ride other mobs in Minecraft
Solution 1:
There is now. You can use the summon command to make Mobs ride other mobs. For example to put a Creeper on top of a Bat:
/summon Creeper ~ ~1 ~ {Riding:{id:"Bat"}}
The more things you stack, the more complex the command becomes. To simplify this I made a summon command generator that lets you stack mobs, but it gives you all the other options too. It's available here.
Solution 2:
You can, with McEdit, but not yet in-game. Here is a video tutorial on how to do something like that.