here's a question?

[A] There are several expressions of the form sick as a ..., that date from the eighteenth and nineteenth centuries. Sick as a dog is actually the oldest of them, recorded from 1705; it is probably no more than an attempt to give force to a strongly worded statement of physical unhappiness. It was attached to a dog, I would guess, because dogs often seem to have been linked to things considered unpleasant or undesirable; down the years they have had an incredibly bad press, linguistically speaking (think of dog tired, dog in the manger, dog’s breakfast, go to the dogs, dog Latin — big dictionaries have long entries about all the ways that dog has been used in a negative sense).

At various times cats, rats and horses have been also dragged in to the expression, though an odd thing is that horses can’t vomit; one nineteenth-century writer did suggest that this version was used “when a person is exceedingly sick without vomiting”. The strangest member of the set was used by Jonathan Swift in 1731: “Poor Miss, she’s sick as a Cushion, she wants nothing but stuffing” (stop laughing at the back).

The modern sick as a parrot recorded from the 1970s — at one time much overused by British sportsmen as the opposite of over the moon — refers to a state of deep mental depression rather than physical illness; this perhaps comes from instances of parrots contracting psittacosis and passing it to their human owners.

At various times in history other animals have been replaced in that idiom. Some theorize that the prevalence of "dog," however, comes from a negative connotation once given to dogs. They were sometimes seen as unpleasant or undesirable.