I believe that the Minfernal shares a spawn with the Tainted Rat. The rats usually spawn as regular critters. Killing them causes another to spawn almost immediately in the area with a small chance to spawn as a battle pet and an even smaller chance to spawn as a Minfernal. This explains why a server reset does not result in a Minfernal spawn. I also noticed that the wolf on the West will come down and kill a rat, leading to a very small "natural" spawn chance. Verification is needed.
1722 pets
I believe that the Minfernal shares a spawn with the Tainted Rat. The rats usually spawn as regular critters. Killing them causes another to spawn almost immediately in the area with a small chance to spawn as a battle pet and an even smaller chance to spawn as a Minfernal. This explains why a server reset does not result in a Minfernal spawn. I also noticed that the wolf on the West will come down and kill a rat, leading to a very small "natural" spawn chance. Verification is needed.