I
retort.
The world has never had, nor has now, "horse-sized chickens." Nobody ever saw a dinosaur or a cassowary and mistook it for a
chicken.
A "chicken" is a very specific thing. According to the all-knowing (though lesser-than-E2) collection of
knowledge called
Wikipedia: "
The chicken (Gallus gallus domesticus) is a domesticated subspecies of the red junglefowl originally from Southeastern Asia." The dinosaur, by contrast, is not a domesticated subspecies of such red junglefowl, nor are any of the other extra-large flightless birds known to adorn many continents. Even a turkey is not a chicken.
The first line of the syllogism being attempted in implying otherwise would be that "all birds are chickens" -- but that is no less fanciful than insisting "all mammals are horses," under which it might be claimed that whales are simply big ocean-roving horses and moles are tiny burrowing horses and wolverines are vicious little motherf--, ahem, carnivorous horses. And humans? Well then, humans are simply hairless bipedal horses. And if humans are a kind of horse, then a fight with a hundred chicken-sized human horses might bode ill, since they would be apt to apply their human intelligence to the problem and outmaneuver you.
But supposing that a horse-sized chicken is literally taken to mean an animal anatomically a chicken but the size of a horse, that is the foe to pick, due to (drumroll, please) the inverse-square law.
The largest chicken ever recorded is this fellow, Merakli, a Kosovar native tipping the scales at 17 pounds. But the average horse weighs over 50 times as much. An anatomically correct chicken which was the size of a horse by any measure (weight, height, length) would be unable to support its own weight on its chicken legs not designed to carry such a load. If it walked at all it would be with immense difficult, and might immediately collapse under its own weight. The "fight" would be a matter of taking a few steps back and waiting for this creature to experience heart failure.
Update: The node below is surely worthy of a full-length retort of its own, but I have but two words: Dik-dik.