If a companion dies during the battle, it will need to be resurrected before it can participate in another encounter or be summoned as a traveling companion. Get that pet a medic! Or in this case, a Stable Master. Stable Master NPCs can heal and revive your pets for a small fee, and there is no cooldown on the Stable Master's heal. You’ll find Stable Masters in major towns and cities across Azeroth.
Learned this today: posted a comment, came back later to see if there was any feedback, reloaded the page: the page posted a copy of my comment. That was not what I wanted. I just wanted to check if my comment was posted and if there were some responses. I did not want to post a copy of my comment from 5 hours ago. And, a humble guess, I think the majority of the users are not expecting such behavior either.