ars_nouveau-1.20.1-4.12.4-all.jar
Forge 1.20.1
When the Wilden chimera takes too much damage it will get stuck in it's second phase forever and will not die under ANY circumstances.
/kill and teleporting it into the void will not get rid of it.
I discovered this when using draconic evolution's mob grinder and I was lucky to also be using rftools dimensions which allowed me to create a new dimension to teleport it to which I could then delete from the folder.