I-Chaya, a domesticated Vulcan sehlat
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. (TOS: "Journey to Babel")
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. (TAS: "Yesteryear")
Background Information
I-Chaya was partly based on D.C. Fontana's cat named Bobby McGee, a reference to the song "Me And Bobby McGee". ("Yesteryear" text commentary) In 1974, Fontana explained, "As to who and what I-Chaya would be as a character, I decided he would be closely patterned on my large cat, Bobby McGee. While Bobby is not old and fat, he has the same affection, snuggle-ability, the fastest claws in the West, and complete disregard for orders. 'One word from me, and he does exactly as he pleases.' The statement fits both Bobby and I-Chaya." (Babel #5; Enterprise Incidents, number 11, p. 27)
The animal is commonly referred to as "Eye-chi-ah" in the final version of "Yesteryear", although the pronunciation that was originally intended is "EE-chi-ah" and "Yesteryear"'s script phonetically spells the name in that way. The name's pronunciation was changed during recording of the episode's vocals, initiated with an error that the production staff then tried to cover up by changing all readings of the name to "Eye-chi-ah." D.C. Fontana much preferred the original pronunciation, referring to it as the "proper pronunciation" and remarking, "The name – always, forever, and correctly – is 'EE-chi-ah.' The other pronunciation is awkward, 'Lumpy,' lacking smoothness, and unpleasant to the ear. In short, it is not Vulcan." (Babel #5; Enterprise Incidents, number 11, p. 27) By way of clarifying how the sehlat's name was to be pronounced, the novelization of "Yesteryear" (in Star Trek Log 1) commonly spells the name as "Ee-chiya".