The ponies, which were pulling a wooden carriage with their owners on board, were likely startled by firework "snaps" and tennis balls that may have been thrown into the parade route by children, according to the Elm Grove Police Department.
Although attempts were made to stop the ponies from running, police added, the carriage entered a crowd of spectators.
A 77-year-old man struck his head after he was knocked to the ground by the ponies, police said. He was not seriously injured and refused medical transport. A 9-year-old boy was transported to a children's hospital after he received minor lacerations to his ear and experienced stomach pains where the carriage struck him.
A 58-year-old woman injured her ankle, shoulder and head after she was struck by the carriage and knocked off her chair, the department added.
The ponies were brought to the parade from Erin Meadows Farm in Hartford, Wisconsin. After the event, they were taken to a veterinarian, ABC Milwaukee affiliate WISN reported.