Configuring Mo'Creatures can be tricky, I've had more luck with editing
the files in the CustomSpawner folder, rather than the MoCreatures
folder. Don't try to change the settings while on the server, the
Mo'Creatures option menu is for single-player only. Without getting into
a lengthy explanation, to change settings: close everything, edit the
config file in the server directory then copy this file to all the
clients.
If you are just looking for some animals to add to your world, I would suggest LotsOMods by TheLarsinator.
It adds animals, not monsters. This mod is a bit simpler, less features
but less options to configure. Plus it has PENGUINS! XD