I'm assuming the reference to the mod is creature_adventurecraft, which is what I named the file that adds the four new monsters.
And first thing I see is the travel advisory bug. If I recall it's somewhere in the announcements file, in the init folder. Apparently code for that announcement was removed when it got changed to display the "reason you can't currently fast travel" statement on the fast travel screen.
Other than that, I'm not sure what else might be causing problems. Though for generated creatures they shouldn't be citing essence materials because only megabeasts are supposed to have them. >.<
EDIT: Oh wait, it is also derping out over my having dummied out the humanoid joints and upper spine body tokens, due to having replaced them with joint definitions on the body tokens themselves.
EDIT 2: I am however spotting miscellaineous mistakes on my part to fix. Mantises reference the wrong name for tanned chitin, giant brown recluse spiders being set to drop venom sacs from their non-existent stingers (instead of mouth), gargoyles having the hoof material template but not the hoof tissue template...
EDIT 3: Ahah, essence error spam is because I retained my outdated method of implementing megabeast hearts that left references to the MEGAHEART material in every creature, when I could instead be citing it only in the entries for the creatures that need them. >.>
EDIT 4: Fix is up, but I still have no idea what would've caused a game crash.