Humans, some non-human primates, guinea pigs, bats, capybara and some birds and fish have lost the ability to synthesize vitamin C - apparently due to a gene mutation defect. Because vitamin C is required for a range of essential metabolic reactions, these animals can't maintain health over sustained periods without obtaining the vitamin by eating other lifeforms (or derived products) which can synthesize it.
“Most animals synthesize vitamin C but some have lost the ability to do so. Those animals that lack vitamin C synthetic ability do not bear any phylogenetic relationship to each other, implying many independent mutations all resulting in the same phenotype. No common environmental influence is apparent. To date, there is no satisfactory evolutionary explanation for the apparent random loss of vitamin C synthetic ability. It remains possible that other animals have not been recognized to have lost the ability to synthesize vitamin C. Identification of all non-synthesizers possibly could enhance recognition of a pattern, but so far none is evident.”
“Vitamin C is by its chemical nature an electron donor, commonly called an antioxidant. However, the widely held assumption that vitamin C has an important role as an antioxidant in humans is unproven.”
Source: Vitamin C: the known and the unknown and Goldilocks Open access article at Oral Diseases journal.