Star Trek: The Animated Series Episode - Yesteryear

Young Spock's beloved pet sehlat named I-Chaya dies in Star Trek: The Animated Series Episode - Yesteryear.

"Loss of life is to be mourned but only if the life was wasted. I-Chaya's was not." – "Cousin Selek" to young Spock, 2237 ("Yesteryear")

In 8877 of the Vulcan calendar (2237), Spock decided to perform his kahs-wan ordeal prematurely, trekking through Vulcan's Forge to the L-langon Mountains. An eager I-Chaya insisted on following Spock, despite being given orders to the contrary. This turned out to be fortunate when a le-matya (a dangerous animal native to Vulcan) attacked Spock, prompting the sehlat to come to the boy's defense. I-Chaya fought the creature as best he could, becoming mortally wounded in the process. Both he and young Spock therefore had to be rescued by the adult Spock, who, posing as his younger self's cousin Selek, had traveled into the past to prevent his own premature death.

Young Spock managed to locate a Vulcan healer to help I-Chaya (and, in so doing, successfully completed his rite of passage). Sadly, the sehlat's condition was beyond hope when the healer arrived, as he had been poisoned by the claws of the le-matya, a deviation from adult Spock's memories of the original timeline.

Faced with the choice of allowing his companion to die slowly and painfully, or giving him a peaceful release from his suffering, young Spock logically decided to have I-Chaya euthanized. Spock's parents later had I-Chaya's body brought back from Vulcan's Forge, to be buried in a place of honor on the family property.