The reason why some animals might look the same whereas you think people look different is that you have a special part of your brain which has evolved to recognise faces with an amazing level of accuracy. Its posh name is the “fusiform gyrus”. Evolution has given us this ability so that we can recognise other faces really well. Our brains find it easier to tell the difference between people as opposed to the difference between cows because we are evolved to do this for our own species the best. It comes more naturally to us to spot the differences in people. But we can learn this skill over our lifetime too as the brain has an awesome ability to do this. Let me give you an example.
I have two cats, both the same breed, english shorthair, both with the same mother, both ginger tabby cats. They look pretty similar but I can tell the difference quite easily between the two cats, not just by their markings. They have different faces, one has big eyes the other has smaller more inquisitive eyes. They both walk differently and they both have different shaped ears. I can tell the difference easily between my two cats, but I have had to learn it.
Animals of the same species do not necessarily look the same (eg. dogs are all the same species).
Additionally, it’s all about perception. All birds of a species might look the same to us, but to female birds choosing a male to mate with, they probably see huge differences in certain features like brightness of plumage, length of tail etc. Some birds have blue colouring that is in a UV range we can’t detect. This colouration depends on how healthy the male is (nutrition and genetics) and the females can definitely tell the difference between various males.
Finally, humans are highly visual (we depend on vision more than other animals). therefore other animasls may dicriminate based on things we can’t perceived (eg: specific pheremones, smells).