A carriage horse keeled over and died in midtown Sunday morning - and animal rights activists have saddled up for another fight against the buggy business.

The stout white equine collapsed on W. 54th St. near Eighth Ave. as it trotted toward Central Park to begin its shift about 9:30 a.m., said Stacy Wolf, chief legal counsel of the ASPCA's enforcement unit.

"The ASPCA has long believed that carriage horses were never meant to live and work in today's urban setting," Wolf said."A healthy horse doesn't just die on the street," said Elizabeth Forel of the Coalition to Ban Horse-Drawn Carriages. "We are asking for an investigation."

The ASPCA said a necropsy will be performed to determine the cause of death.

"It's a tragedy," said a spokesman for the Horse and Carriage Association of New York. "It's not something that happens regularly. ... Our horses are taken care of."