Vitamin C has many functions in addition to the prevention of scurvy and these functions require it in gram, not milligram, quantities per day. In fact, vitamin C is required in larger quantities than any other vitamin. It is a macro nutrient like calcium and the essential amino acids, not a trace nutrient like most other vitamins. This includes cats, dogs, horses, cows and virtually all other mammals except the primates and guinea pigs as well as virtually all birds, amphibians, and reptiles. All animals tested to date synthesize vitamin C daily at a rate equal to between 2 g and 10 g per 150 pounds of body weight despite the fact that their diets include at least traces of this vitamin.
Most animals get most of their other vitamins from their food just as humans do.
Like humans, they aren't set up by nature to synthesize most vitamins because the quantities in their food are sufficient. There are, admittedly, a few animal species that do not synthesize vitamin C within their bodies. These are a fruit-eating bat from India and a few Indian birds as well as guinea pigs and primates. A tropical diet, especially one rich in fruit, yields exceptionally high quantities of vitamin C macro quantities, not just trace quantities. If an animal outside the tropics is born with a genetic mutation which prevents it from synthesizing vitamin C it will be come sickly because it cannot get the large quantities that it needs of this nutrient from its food.