This idea comes from an extraction shooter I played recently and they had robot dogs that followed you and were able to carry items for you in the raid so you could get more loot to carry. But I think it would work here too as an actual dogs. Having a companion dog which you could order with commands like stay, follow, maybe attack only for AI, go home. Dog would have it's own pouches, small sides backpack(which of course you would need to find in-game) to hold your gear and follow you around and maybe help with guarding you against AI , but not players, because I think that would be too annoying and too overpowered in my opinion. This would be really cool feature, but either you would need some kind of special occupation or skill to be able to have the dog trained, which would have some other downsides to be balanced and it would be rare or very expensive to keep, since you would need x2 food, for yourself and dog to maintain. Maybe different type of dogs like German Shepperd could guard you against AI, and like Labrador Retrievers would be like carry items only dogs with limited capacity backpacks on them. 
#Companion
gloomy lily
For balancing purposes:
German Shepherd: Role Guard+light carry maybe just 4 slots, so it can carry at least, its own food when needed or some other not big items. Guard effectiveness against AI only high(distracts 2-3 zombies). Maintenance cost high(Premium dog food). Unique downside high noise level: aggressive barking alerts players.
Labrador Retriever: Role pure carry has 12-16 slots. Guard effectiveness against AI zero. Maintenance cost medium(needs standard dog food). Unique downside low stamina: slows you down after you sprint too much non stop for too long.
raw gyro
gloomy lily
I think for PVE/singleplayer experience it would be so much more fun to have a dog to take care of and not feel that lonely/solo. Same for multiplayer for solo players would be something more to have. Maybe make it not findable and it just spawns with an occupation like "Dog trainer" where you could choose dog type etc.
agile copper
You forgot to add a pug. Has no guarding or carrying capacity, but helps keep you sane and provides comfort and emotional support. Must be carried around in a carrier on your chest like a baby when travelling on foot. Maintenance cost: Will eat any dog food or even people food and consumes far less than the other dogs, but does require you interact and play with it often to stay bonded or it will run away. Unique downside: due to being carried it takes up 20lb of your carry weight.
gloomy lily
Well it's up to developers to decide what companions they add and decide if it's even needed in the game in the first place. I just gaved exampled and suggestion thats it.
agile copper
same here 🙂 its just a different type possibility
loud delta
+1 on this
manic citrus
Definitely need a dog for the end of the world.
trim fulcrum
TL : DR = I love your idea 👍
Dono if Anyone here played « Fear the Night » it’s a deadgame in the egg but was pretty cool (hundred’s of hours on it) you were able to « hack » robots to carry stuff as you discribe And some others was able to fight with/for you or défend your base
Like in Ark but instead of dino it was badass robots