What is the origin of the idiom "sick as a dog/horse" and when to use it?

The idiom "sick as a dog/horse" has origins that date back to the 17th century. It is believed that the phrase emphasizes the perception that dogs and horses can become quite ill and display symptoms of sickness more visibly compared to other animals. It is used to describe someone who is very sick or experiencing extreme illness or discomfort. It conveys the idea that the person's sickness is severe, similar to the condition of a sick dog or horse.