Zoo officials say a hippo named Samantha has been put down.
The animal was suffering from an incurable kidney condition and ulcers.
She was 24. Samantha arrived from the Cheyenne Mountain Zoo in 1980 as a one-year-old calf. She later gave birth to 15 of her own calves, many of whom now live at other zoos.
Zookeepers say they had noticed a change in Samantha's behavior over the past two months.
Parts of the animal, like her hide or teeth, may be put on display for educational purposes.