The first time I had to do this was with my big red cat, a Maine Coon mix who'd shared my life for more than 16 years. As the doctor very gently came to give him that last shot, he turned his big shaggy head and growled. At her. Courage and gall to the last. His ashes are up in the Colorado mountains; I figured he'd always thought he should have been a bobcat.
I came home that evening and couldn't relax. The other cat hustled out and leaped into my lap, like "I'm the star now!!" It helped. But I miss the big red guy still.