I'm not sure what the context of this image is, but it was posted by No Kill advocate Nathan Winograd today in honor of the 125th anniversary of Henry Bergh's death. Bergh was a tireless advocate for exploited and ignored Nonhuman Animals in urban areas.
Notice the African American man in the right hand side of the crowd. He appears to be gesturing in support of Bergh's attention to the horses. According to the notes at the bottom, this drawing appeared in Harper's Weekly. Judging from the dress and the time when Bergh became active for Nonhuman Animals, this scene would have transpired just after the emancipation of African American slaves.
It is likely that the artist intended to demonstrate how Nonhuman Animal suffering mirrored the suffering of many humans. Indeed, many early Nonhuman Animal activists drew heavily on the claimsmaking of the abolitionist anti-slavery cause, recognizing how oppression worked similarly across many vulnerable groups.
See Diane L. Beers. 2006. For the Prevention of Cruelty: The History and Legacy of Animal Rights Activism in the United States. Athens, OH: Ohio University Press.