According to some redditors, these are Chilean police officers. Their horses are armored, like those ridden by medieval knights. But the armor is thoroughly modern. A 2012 article from the Chicago Sun-Times describes that city's police horse armor:
The solicitation specifically requests: leather nose guards — 22 inches long and five inches wide — with “impact-absorbing foam”; clear, wrap-around visors to protect the horse’s eyes; 17-inch-long rear leg shields and 11-inch-long front leg shields, both made of “high-impact plastic” and soft foam; and “training aids” for horses, including a “crowd control training ball set.”
The mounted unit has 30 horses, 30 officers and an annual budget of nearly $2.7 million.
Police Department spokesperson Melissa Stratton said all 30 horses will be equipped with the new riot gear. She noted that the horses are “great crowd control tools” expected to provide “significant support to officers on the ground” during the summits.
Reddit Thread and News Article -via Super Punch