10. Gus the Polar Bear
In 2013, he had a poor appetite and experienced difficulty swallowing food. In an attempt to determine the source of the problem, he was placed under anesthesia and veterinarians eventually uncovered a large, inoperable tumor near his thyroid. He was consequently euthanized at the age of 27.
Gus the polar bear was another icon of the Central Park Zoo who was visited by over 20 million people during his lifetime. Throughout the 1990s, he was the face of the zoo and was prominently featured in advertisements and promotional posters. However, in 1994, visitors noticed that Gus had developed obsessive behavior, swimming laps in figure-eight patterns for up to 12-hours a day. Patrons gave him nicknames like the “bipolar bear,” calling him neurotic and flaky. As part of his treatment, he was given Prozac, making him the first animal in history to be treated with the medication. Zoo officials attributed his abnormal behavior to depression, which was thought to have developed following the death of his partner, Ida.