The current understanding of the way dogs think:
Dogs have some sense of “fairness,” but it doesn’t seem to really care about “equality.”
So for example, if you have 3 dogs, and you give one dog a treat and the two nothing- the two who got nothing will see it as “unfair.” If you give one dog a whole treat, and the other two dogs get a half treat- because they all got something- they will all see it as “fair.”
What is interesting there- is that let’s say you have to give one dog a vitamin, or even some nasty medicine from a dropper- and the other dog seems upset or is trying to get some too- you can give one dog nasty medicine and one dog a treat- and both dogs will usually see it as fair! The treat dog will assume the other dog got a treat, and the medicine dog will assume the other dog got medicine.
Now- anecdotally- some people may point out that when one dog gets a larger helping, or one dog finishes their treat faster- they may seem like they are upset or can tell it isn’t “fair.” Evidence suggests that this isn’t because they can tell it is unfair quantities- it’s because they see the other dog eating RIGHT NOW, and they are not eating RIGHT NOW- so regardless of how much they JUST had or if it was equal or even greater to then portion of the dog who isn’t finished yet- they will feel like they are being shafted because they see the other dog eating and they are not eating!
Of course- we have enough trouble understanding human cognition let alone applying a cognitive model that covers all humans, and that is not just very pressing to humanity but also a larger potential source of profit than understanding the minds of other species. So our understanding of how animals think and feel is flawed- as is our ability to understand other species without relating the way they think and behave to how we feel and think.
Dogs have some sense of “fairness,” but it doesn’t seem to really care about “equality.”
So for example, if you have 3 dogs, and you give one dog a treat and the two nothing- the two who got nothing will see it as “unfair.” If you give one dog a whole treat, and the other two dogs get a half treat- because they all got something- they will all see it as “fair.”