Only a few species, including humans and guinea pigs can't produce their own vitamin c. Dogs, goats, whatever under stress or illness make it. That is part of why animals can be healthier.

Lack of vitamin c genes would be a good code of control and a strange evolutionary choice.

Some fruit eating animals also don't produce it because their duet is so rich in vitamin c fruit.

I think we could be hybrid from sea mammals. Check out the Aquatic Ape theory, our unique similarities.