Zoos are incredibly unlikely to have their own MRI machines because the cost is prohibitive, and other imaging technology (X-rays and ultrasound) is perfectly adequate.
Since zoo animals have closely-monitored diets, are kept in controlled environments away from predators, and don't have to fight for mates and territory, most of their health issues are pretty straightforward--dental infections and swallowing foreign objects are common; in elderly animals, it's arthritis and cancers. But because most zoo animals are still at least semi-wild, there are serious limits when it comes to treatment options, and having more detailed images of what's wrong doesn't change that.
So zoo vets not only have virtually no need for an MRI to diagnose and treat their animal patients, they can't order lots of extra imaging and bill that shit to insurance in order to pay for the machine, as human hospitals do. So an MRI machine of any size would be an immense waste of money for a zoo--it's money that is far better spent on upgrading exhibits and educational programs, and improving day-to-day animal care.
Veterinary teaching hospitals might have MRI machines just big enough to accommodate a horse (and we're talking a Thoroughbred, not a Clydesdale), but zoos? No. There is no hippo- or elephant-sized MRI that can accommodate the Boogie- or Chance- or Slaton-sized eatbeasts who have eaten their way out of the largest hospital MRI.
There are plenty of urban legends about obese people being sent to the zoo to use the MRI built for hippos and elephants, and they are just NOT TRUE. They're not. At the risk of being powerlevelly, I have some firsthand experience behind-the-scenes at two large zoos known for their high level of animal care, and neither one had an MRI, or any plans to get one--and for all the reasons I've stated.