As an addendum, I was unable to reproduce this in the most controlled setting available to me: “testing my defense team” composed of four Orc Summoners. The trait-spawned Ancient Horrors appeared at mythic rarity with all appropriate levels and stats. I will come back again the next time the trait activates to confirm whether it has resolved itself, but this at least was a persistent problem.
Edit: @Ozball, just explored in Karakoth until I got the trait to proc. The Ancient Horror was back at level one, while the magic of the Warlock was much higher. I can take screenshots if it will help. Trying to reproduce this in PVP is too difficult, since there are far fewer common troops with that trait (and since default stats are higher so it takes longer to whittle the opponent down).