I just read this and had it in mind as I set out to do my "3 Spiketail Beavers defeated with Snowfeather Hatchling" quest.
I came upon a Black-Footed Fox Kit and decided to clear it, battled it, and there was a Spiketail Beaver as backup.
Despite defeating that team with my Snowfeather, though, I didn't get credit, of course.
P.S. Second battle. Spiketail Beaver, with another Spiketail Beaver as backup. Again Snowfeather in my team. First pet. Also came in to kill the final pet. Also Summoned throughout the battle and still there at the end. Again, no credit.