This Chest companion is not like the basic minecart chest on a wolf but a little better i guess ?
Spent quite a long time on this trying to get the mobs to work correctly. 
It has 15 inventory slot of course having a donkey on the top. Good companion I must say :D

Command:
/summon EntityHorse ~ ~1 ~-1 {Type:1,NoAI:1,Silent:1,Attributes:[{Name:"generic.maxHealth",Base:1}],Invulnerable:1,Tame:1,ActiveEffects:[{Id:14,Duration:1000000000,Amplifier:-1}],invisible:1,ChestedHorse:1,CustomName:"Chest Companion",Riding:{id:ArmorStand,ShowArms:1,Invisible:1,NoBasePlate:1,Small:1,DisabledSlots:1,Equipment:[{},{},{},{},{id:54}],CustomName:"Chest Companion",CustomNameVisible:1,Riding:{id:Wolf,Invulnerable:1}}}