The poster child in the veterinary world for a Vitamin C deficiency is the guinea pig. The Caviidae family of rodents (also see the largest rodent a Capybara!) do not have the ability to synthesize Vitamin C in their bodies, like humans! This is because Vitamin C cannot be formulated in the body due to a lack of a specific enzyme (gulonolactone oxidase, if you're interested).
Since I have been seeing a lot more exotic pets, I have seen a few clinical signs of Vitamin C deficiency in the guinea pig. You may see lameness and/or curly nails. Nasal discharge and dental issues, muscle wasting despite a chubby belly, poor digestion, necrotic penises. Just with every veterinary visit, a history of the animal's nutrition is important. I ensure that I check to see what the guinea pig is consuming. Not just what it is being offered! Inappetence, or a reduction in the amount of food that your guinea pig is eating, is a very non-specific clinical sign. It really could mean anything. The history you collect from your pet owner is going to be the ticket along with your physical exam.
Guinea pig pellets are fortified with Vitamin C. But the Vitamin C degrades over time, so really, if your guinea pig doesn't consume that bag of pellets within 90 days, it's not good enough.
The Oxbow Vitamin C chewable treat is 25 grams of Vitamin C. So if you have a 1 kg guinea pig, then one of these chews should suffice (if they are healthy and have no nutritional deficiency). For the ill guinea pig, or those that were deficient, they will need more vitamin C. As a rule of thumb, it is 10 mg/kg/day Vitamin C for a healthy adult guinea pig, and 30 mg/kg/day for a growing/young or pregnant/lactating guinea pig. Bump that up to 50 mg/kg/day for while they are ill.
If your guinea pig does not enjoy the chewable treat, then ensuring there is enough vitamin C in their daily salad will be important. See this handout on the amount of Vitamin C in some food items that you can feed your guinea pig. BUT one thing to note is that you also do not want to feed a low of high oxalate vegetables to your guinea pig, as this can lead to calcium oxalate bladder stones. This is a good list of veggies and their oxalate content. Choose things that are lower on the list to feed your guinea pigs (cilantro, kale, pepper) and limit the amount of high oxalate foods (spinach, parsley, beet leaves).
If you're not really interested in exotic species, then Vitamin C won't be that interesting for you, because dogs, cats, horses, cattle and pigs can all make Vitamin C. However, if you see a question on your board exam about guinea pigs, you may wish to remember this one.