Sehlats are kind of like dogs - but larger, with bigger fangs. Vulcan children have them as pets. I-Chaya was Spock's when he was a kid.

I-Chaya died protecting Spock during his kahs-wans when spock was 7. Kahs-wans is the vulcan coming of age ceremony where little vulcans have to go into the dessert and survive by themselves for a number of days.