Supposedly, this was a mistake.
Taken from:
https://www.2001italia.it/2013/12/how-did-they-shoot-leopard-scenes.html
"Every cat owner knows that the eyes of the leopard glowed because of a tissue in the eye that reflects visible light back through the retina, increasing the light available to the photoreceptors, contributing to the superior night vision of the animal. As the scene was shot using the Sinar front-projection system that used 8x10 inches transparencies on a 110-feet-wide screen covered by 3M reflective material, the eye of the leopard worked just as the reflective material of the background."
