Arguments take into account raising grain to feed to the horse - with a tractor - but if they'd use the horses to till the soil for their own feed grain too, the carbon footprint would effectively be even lower.
Of course, there's an even easier way to visualise it. Did the process that our forefathers used, with a horse and plough, create tons of pollution every year? (Don't count manure because that is a fertiliser and some of it is recycled into the same grains that then feed the horses.) Then, compare to the modern method - do tractors produce tons of pollution every year? Assume a few hundred acres, times two operations per year at least (seeding and harvesting) gives me a figure of around 20 tonnes of carbon for tractors and 4 tonnes for a horse.
It's the right way to look at everything food-related we do. Did multiple small farms, each growing a spectrum of crops, vegetables, and animals, cause large-scale environmental damage? Did those small farms result in ideal breeding conditions for particular monoculture pests that then require tonnes of pesticides to control? Of course not. Mixed farming is very eco friendly. Monoculture farming (growing an entire farm - and sometimes district - worth of the same crop ) results in pests taking advantage of the sea of food.
An apple orchard can lose the whole crop (or sometimes even the whole orchard's worth of trees) to a common borer. Spraying for the borer creates direct pollution from the chemicals used, indirect pollution due to transport and application of that chemical, and the processing needed to manufacture the chemical. If the apple orchard were interplanted with other fruit trees and crops, the losses would almost never get above 15%. Now that's my kind of QED...
Same with animals on farms. If you have miles upon miles of chickens, then you need to bolster them with antibiotics, supplement their feed with chemicals. Those same chickens spread across several hundred farms would need almost no attention, and instead of occupying expensive sheds, would actually contribute to the farm by their manure and their scratching and weeding and pest control.