Minecarft 1.16.5
Forge 36.0.48
AN 1.11.6

See console spam here as this is recognized by Darkhax mod Bad Mobs.

I do not know what Bad Mobs is, but that entity is registered the exact same way as the rest of the mobs in the mod:

I've posted to a similar bug report on Bad Mobs in hopes of the two of you being able to figure out what/why it's happening. Find their issue report HERE

I see the same issue with BadMobs entities. BadMobs allows server owners to disable specific vanilla mobs from spawning. So if I didn't want creepers or Ghasts, I could update the config file for BadMobs and chose to disable them. Same issue reported in Darkhax-Minecraft/BadMobs#46

Sorry for the issue. This was caused by a bad update to forge/badmobs. The config spec is being generated at the wrong time so all non-vanilla/properly registered modded mobs are being reported as being incorrectly registered. You can safely disregard this issue.

Thanks to Darkhax. It's unrelated to AN so I'm closing the issue