Which of the following is a common sign of internal parasites in animals?

Vomiting is a common sign of internal parasites in animals due to the irritation and damage these organisms can cause within the gastrointestinal tract. When an animal is infected with intestinal parasites such as worms, the disruption in normal digestive processes can lead to nausea and subsequent vomiting. This response is the body's way of trying to eliminate the parasites and manage the discomfort they cause.

In contrast, the other options—weight gain, clear eyes, and a bright coat—are generally associated with good health in animals. Weight gain, for example, is typically a sign of adequate nutrition and absence of disease, while clear eyes and a bright coat are indicative of a healthy and well-cared-for animal. Therefore, these options do not correlate with the common symptoms one would expect to see in an animal suffering from internal parasite infestations.