Nice looking pair of horses! To answer a previous question, "Little Oak" is the model of tractor built by the Willmar company. In 1917, when the Little Oak was built, there were something like 200 companies building tractors in the US.
A few more pictures from the George Schaaf collection George Schaaf has been collecting and restoring classic tractors since the 1980s. (Mecum Auctions)
The Gas Traction Company Big Four 30 was the largest tractor of its day. (Mecum Auctions)