A few notes:
A variation means that an enemy returns in a new form and operates at least somewhat differently but still largely the same. The panzer in DE is slightly different from the panzer in Origins due to its claw being replaced with a canon. Alpha Omega’s lightning dogs count as reuse instead of variation since they are functionally the same as normal Hellhounds.
Overall, enemy reuse went up in later games starting with BO3 with a peak in BO4. However, new enemy creation lowered in the last two entries which contributes to the community getting tired of enemy reuse.
BO3’s reuse numbers are inflated by Revelations which contributed to 5 out of 11 reuses and brief appearances of electric/fire zombies which contributes to 4 out of 11 reuses.
BO4’s reuse numbers are inflated by catalysts which contributed to 13 out of 24 reuses. Fire catalysts did not contribute to the new enemy count since they are very similar to Napalm zombies
BOCW and BO6 had their numbers inflated by armored and heavy zombies but I felt they should be included in the count as Catalysts and electric/fire zombies were counted in other games.
I did not include enemies spawned in by elder disciples or vermin only spawning in middle of rounds as reuses for BO6. Outbreak was not included in the counts for BOCW.
Der Riese Hellhounds count as a variation since they function differently from Shi No Numa Hellhounds in regards to how they affect health regeneration