The sign does not indicate that horse meat will be served. The meal is for the horses.
The American Society for the Prevention of Cruelty to Animals (ASPCA) was founded in 1866 after Henry Bergh saw a cart horse being beaten. In those days, horses were for work, hauling people and all the goods they required. The ASPCA threw Christmas parties in many metropolitan areas where people gave little thought to the workhorses around them. A typical party would feature a tree festooned with corn or apples or something a horse would consider a treat. The city's working horses would find crates of carrots and oats as well. These parties were public, and people were invited to interact with the animals and maybe get a treat for themselves.
The purpose of these parties was not only to give the horses a good time, but to introduce the horses to the public as workers deserving of good treatment. The ASPCA would promote better treatment for working horses and all animals at the same time. The men who drove horse carts were included as well, as they needed a break from work and they understood their horses better than anyone. At least some them did- those who were cruel to their animal charges could get a lesson in humane treatment. The ASPCA's horse parties continued until urban horses were completely replaced by motorized vehicles, but hung on some areas longer than others. Chicago held such parties up until the 1960s! Read about the ASPCA's Christmas parties for horses at Smithsonian. -via Strange Company
(Image source: Library of Congress)