The horses, part of the Chicago Police's Mounted Unit, were let out of their stables sometime between 9:45 and 11:15 p.m., said Lt. Paul Bauer.
One of the horses was apparently sprayed in the face with a fire extinguisher. A second horse had a cut to its leg.
Twenty-seven of the nearly 30 horses were released from their stables and were found walking around the barn, which had tissue paper strewn about.
"At this point, it's all conjecture as to how they got in, but obviously there's people out there who would know who entered this stable, and they did not enter this to commit a theft, rather it appears they entered this to maliciously harm our horses. For what reason, I don't know," said Bauer.
He declined to answer questions about security measures in place at the stables.
"The horses were very agitated and fearful because of what they had just endured," he said. "The type of incident that happened last night upset a lot of people, including everybody that works here."
No arrests had been made as of Monday afternoon but officers continued to investigate. No animals were injured last week in a fire at a stables on the city's near north side that investigators deemed suspicious.
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