/summon is a command that allows you to summon your own monsters. You can make them wear armour, ride other monsters and create various things! Here are a few:
Creeper Riding Bat: /summon Creeper ~ ~1 ~ {Riding:{id:Bat}}
Zombie with my head: /summon Zombie ~ ~1 ~ {Equipment:[{},{},{},{},{id:397,Damage:3,tag:{SkullOwner:Shocker6666}}]}
Skeleton wearing diamond armour named "Boxer" who has 50% chances to drop his armour: /summon Skeleton ~ ~1 ~ {Equipment:[{},{Count:1,id:313},{Count:1,id:312},{Count:1,id:311},{Count:1,id:310}],CustomName:Boxer,CustomNameVisible:1,DropChances:[0.0F,0.5F,0.5F,0.5F,0.5F]}
A rideable EnderDragon: /summon Boat ~ ~1 ~ {Riding:{id:EnderDragon}}
If you want to suggest a mob, feel free to post!
EDIT: If these don't fit in chat, put them in a command block. To get a command block, do /give (your name) 137
Creeper Riding Bat: /summon Creeper ~ ~1 ~ {Riding:{id:Bat}}
Zombie with my head: /summon Zombie ~ ~1 ~ {Equipment:[{},{},{},{},{id:397,Damage:3,tag:{SkullOwner:Shocker6666}}]}
Skeleton wearing diamond armour named "Boxer" who has 50% chances to drop his armour: /summon Skeleton ~ ~1 ~ {Equipment:[{},{Count:1,id:313},{Count:1,id:312},{Count:1,id:311},{Count:1,id:310}],CustomName:Boxer,CustomNameVisible:1,DropChances:[0.0F,0.5F,0.5F,0.5F,0.5F]}
A rideable EnderDragon: /summon Boat ~ ~1 ~ {Riding:{id:EnderDragon}}
If you want to suggest a mob, feel free to post!
EDIT: If these don't fit in chat, put them in a command block. To get a command block, do /give (your name) 137