The thoroughbred named "Mikey C" died suddenly Monday while he and his partner, Officer Paul Cassasanto, were on patrol.
The horse was named after a late Chicago Police officer.
Twenty-seven police horses are housed in the stables on South Lake Drive Shore. The stable belonging to the 18-year-old horse Mikey C is empty.
Mikey C died unexpectedly Monday while on patrol with Officer Casasanto at North Avenue Beach.
"What we believe is probably a heart attack," said Lt. Paul Bauer, who is in charge of the mounted patrol unit.
Casasanto was off Tuesday and was too upset to talk to reporters anyway.
However, Casasanto's colleague, Officer Jason Ayala, who is also a police officer assigned to the mounted unit, was with him Monday while they were patrolling the beach.
"He was distraught, he was upset," said Ayala. "He said Mike was a good horse, one of our lead horses."
"That horse was bombproof," Bauer said. "You could stand in the middle of a demonstration, and nothing would bother that horse."
In 2001, the unit started to name the mounted horses in the memory of fallen officers. Mikey C was named in honor of fallen Chicago Police Officer Michael Ceriale, who was fatally shot while conducting an undercover drug investigation.
It is the police department's way of remembering the sacrifices they made and a way to keep them in their thoughts on a daily basis.
"Not only is the loss of a police horse mourned here, but also the police officer who was killed in the line of duty," said Bauer.
Mikey C and Casasanto worked closely together for eight years. The officers become attached to their horses and form a bond. They become part of their family.
"You are with the animal the majority of your day...You really develop a bond with your house," said Ayala. "I feel for my partner...You learn what he likes and does not like, just like the horse learns about you."
Ayala says it would be difficult to lose his horse, "Kaspar."
The unit is already preparing to replace Mikey C with another horse to match Officer Casasanto with another partner.
Normally, the mounted horses live into their mid-20s.
The police department says the training for the horses can be anywhere from one to six months and the grueling training for the officers is 14 weeks. Many of them do not make it and drop out.