To Reproduce
Steps to reproduce the crash:

1. Spawn Arachnon with egg
2. Punch Arachnon
3. Crash

Crash log
https://hastebin.com/tiwelakiwa.rb

Versions
Forge: 35.1.10
Pandoras Creatures 1.16.3-2.0.1
Minecraft 1.16.4

I relaunched the game without changing anything and I couldn't reproduce it anymore... Odd; perhaps an obscure bug? In any case, seems to be resolved. Thank you very much!

Hm probably a mod conflict, but I cant figure out what mod is causing it, the only mod I see mentioned in the log you posted is astral sorcery, so perhaps could you try removing it and see if the bug is gone? If it is we would know for a fact that its due to a mod compatibility issue. Also try launching the game just with PC installed and see if it happens, if it does the problem is somewhere else.