Quote from MustangDood
There seems to be a new incompatibility with Mo' Creatures newest version, which was released yesterday.
In my own (limited) testing, I've narrowed it down to something in the MoCreatures.zip file that goes in the \mod folder.
BTW will work fine without Mo' Creatures. Mo' Creatures will work fine without BTW. Put them together, and it crashes at the main menu (before you load a world).
Since they both work independently, I'm not sure where the best hope for a fix is.
Anyway, these two mods used to work together in 1.1, so I didn't think it would hurt to bring it to your attention, in case there is something you were interested in adjusting, to make them happy again.
I'm running BTW with Mo' Creatures (and a handful of other things) with no issues