It apparently is the polar opposite of the senior-pet tag.
It was proposed during the Biweekly Topic Challenge by moderator John Cavan, and became a high-voted answer (upvoted to 3 at this time), defined loosely as:
We've had senior pets, but what about the junior-pet scenario, specifically around raising young animals such as kittens, puppies, etc.
During the actual work time of The Junior-Pets Challenge, I was the only participant to put the new tag "out there", doing so reluctantly:
this tag doesn't even have any questions that exist with it. I almost think it is a disservice to generate questions for a nonexistent tag, but I went ahead and re-tagged several puppy/kitten/egg questions with junior-pets, which may or may-not end up being relevant. Also created the tag wiki.
I created the tag wiki to oppose senior-pet as follows:
Addresses specific needs of younger / newborn pet companions, and ways to improve their overall well-being and health.
Your move, Pets.SE community., leaving it up to others to determine whether or not we should continue with the process. No one decided to tag any other questions; I chalked that up to the same lack of participation I've seen during the other challenges, as I had left the discussion floor open in my answer to the challenge.
Obviously it rubs you the wrong way. You're welcome to revert those tag appliques (just make sure you leave any functional changes that might have been made, because I tend to multi-task in my edits).