Step 1: Load Minecraft Version 1.8
Step 2: Spawn In A Horse (Does Not Matter What Kind-Helps To Use A Donkey Or Mule)
Step 3: Type The Command /entitydata @e[type=EntityHorse] {Type:1,Tame:1}
Step 4: Saddle & Chest The Donkey
Step 5: Again Type The Previous /entitydata Command But Replace The {Type:1} With {Type:6}
Step 6:Now You Should Have A White Horse But With The Ability To Hold A "Storage Crate" Like Donkeys!