I'd go with horses because they can reproduce and replace themselves, and tractors can't. You wouldn't necessarily need huge horses, some of the smaller breeds such as the Canadian Horse are excellent multipurpose animals and easy keepers.http://www.chhaps.org/cdnhistory.htm
The Canadian Horse is a true national hero. It is a heritage breed older than Canada itself, descending from the first horses to set hoof on the shores of New France in 1665. Like the habitants who depended on it for their survival, the Canadian Horse has struggled for its survival and identity in the face of mechanization, market forces and acculturation. Long ago dubbed “Le petit cheval de fer” or ‘the Little Iron Horse” because of its ability to out-pull pound for pound, any other kind of horse, the Canadian deserves to be respected, conserved and treasured as a living part of our nation’s history. In April 2002, the breed was recognized, by law, as the National Horse of Canada.
Just what kind of horse is the Canadian? Surviving books and artwork from the past tell us that the Canadian Horse was just like us - a spunky, tough breed unsurpassed for its ability to survive and flourish in harsh conditions. Eighteenth century historian Etienne Faillon, perhaps best captured the image of the breed with his words ``small but robust, hocks of steel, thick mane floating in the wind, bright and lively eyes, pricking sensitive ears at the least noise, going along day or night with the same courage, wide awake beneath its harness, spirited, good, gentle, affectionate, following his road with finest instinct to come surely to his own stable''.
One of the breeders working on compiling a breed history once sent me copies of the historical documentation that she was using. One diary was from a logger who complained that his Canadian Horse stallion was such a hard worker and so strong that the Belgians he used to team with it died of overwork trying to keep up. Justin Morgan's horse Figure, who out-pulled and outran the horses he competed with was certainly a Canadian Horse stallion (although Morgan breeders pitch a fit when you point this out to them). DNA testing by the University of Guelph has confirmed the origin of the Morgan breed as being the Canadian Horse. So, a work horse for your story needn't be a huge hayburner.