Sick as a...?

Where did the idiom "sick as a dog" come from? I dunno, I've been too sick to care. It’s something tubercular… altogether phlegmy and most unpleasant. And worst of all, I can’t taste ‘nothin. I made a big vat of chicken soup before the height of the malaise hit, but I have no idea if it’s any good. In between napping and listening to podcasts, I’ve made a small stack of potholders, photos forthcoming. I also watched several movies: Superbad (pretty funny); Ant Bully (stupid); Other People’s Lives (really interesting and somewhat sad); Steal a Pencil for Me (a little slow but poignant); and two episodes of Dexter (creepy and oddly riveting in a guilty way).

Here's one take on the sick-as-a-dog expression from worldwidewords.com. Is this site not potucational?

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).

So there you go. (sniff)