So in this tutorial, I will be showing you how to spawn Custom Mobs eg:
A super fast EnderDragon with an upside down skelton ontop of it or....
A HUGE PIG WITH 75 BUTTERFLIES CARRYING IT AROUND AND SHOOTING FIREBALLS AT VILLAGERS,
well not quite that cool but I ill be showing you a couple cool things that you CAN make.
So to start with go into creative mode to get a command block, or in multiplayer simply type: /i commandblock 1
Next place it down and paste the following code into the editing part:
/summon EnderDragon ~ ~1 ~ {CustomName:"Dinnerbone",Riding:{id:"Skeleton",Equipment:[{id:391}]}} 
What this will do is spawn a Enderdragon 
/summon EnderDragon
Directly above the command block
~ ~1 ~
That will be riden by an upside down skeleton
{CustomName:"Dinnerbone",Riding:{id:"Skeleton",Equipment:[{id:391}]}}
Pretty easy once you break it down.
So you can actually mess around with this yourself.
Just use this code I made for you!
/summon MobName ~ ~1 ~ {CustomName:"CustomName",Riding:{id:"Mob2Name",Equipment:[{id:ItemInHand2}]}}
Okay so basically,
Start by replacing  MobName with the name of the mob you want, which in my case is the EnderDragon.
Next replace "CustomName" with the name you want for the mob. Just don't delete the quotation marks.
Finally replace Mob2Name with the Mob riding the other Mob and don't forget ItemInHand2 which is for the item in the riding mobs hand, which it will be holding for the ride, required for the skeleton mob.
So in the end we get /summon EnderDragon ~ ~1 ~ {CustomName:"Dinnerbone",Riding:{id:"Skeleton",Equipment:[{id:391}]}}


I hope this tutorial was not confusing for most of you and I hope I really helped enjoy and goodbye :)