I-Chaya was a beloved pet sehlat that Spock had in his boyhood. I-Chaya was quite old and had belonged to Sarek during his own youth.
When Spock undertook his kahs-wan ordeal prematurely, I-Chaya insisted on following him despite Spock's orders against it. It turned out to be a good thing, considering that a le-matya – a dangerous animal native to Vulcan – attacked Spock and I-Chaya.
I-Chaya fought the creature as best he could, but the old sehlat fell in battle and both he and young Spock had to be saved by the adult Spock, posing as young Spock's cousin Selek, who had traveled into the past to prevent his own death at this moment. Although young Spock managed to find a Vulcan healer to help I-Chaya, the sehlat was beyond meaningful aid when he arrived.
Faced with the choice of a painful extended existence or a peaceful release from his suffering, Spock logically chose to have I-Chaya euthanized. It was an act that marked Spock's decision to follow the philosophies of Surak.
Spock's parents had I-Chaya's body brought back from Vulcan's Forge, to be buried in a place of honor on the family grounds.