Beasts From The Darkness is a modpack which adds over 4 new dweller type creatures and other terrifying aberrations.
Makes the game WAY more difficult, adding new features such as Temperature, Thirst and Sanity as new statuses. New Weapons can be made to combat the monsters of the caves, however they will not completely protect you from them.
Early game progression is much more difficult, as trees are unable to be broken by fists, wood and stone tools can not be obtained, and torches and lanterns run out of fuel or can be broken easily.
Good Luck.