Someone has been busy, very busy making creatures. In this setting one of several possibilities have occurred. Some brilliant if possibly mad scientist has somehow made a huge number of creatures. It may be the case that the PCs have encountered a far away land where monsters have always lived. Otherwise many inventors have been hard at work making a myriad different types of monsters that now walk the earth.
In some stories, not all the monsters have to be a threat to the players. They may be created and used by a government to defend its borders. Some stories may even have domestic monsters that were made to do menial labor. This is up to the GM and players as to what flavor of situation should be taken.
Historically medicine of the time is only starting to be tested scientifically. In the steam age things like desires and moods are thought to be regulated by fluid imbalances and that can be tuned. Changing the nature of a creature according to this thinking could be as simple as adding or subtracting the right tissue or fluids.
One possible way there could be such a large number of creatures is if they self replicate. Tiny creatures may be able to replicate in moments, larger creatures are likely to take days of incubation. How this process works is up to the GM but the speed and number of times a monster replicates should be balanced with how dangerous the creature is.
Newly Discovered Lands
Monsters may inhabit another land that the PCs have discovered, especially if other seeds like Primitive Society or Untamed Wilderness are selected. In this situation a iguanadon may be as common as deer in the forest.
An alternative is that something in this new land, mineral, gas or malevolent man, has made ordinary animals or human inhabitants into monstrous creatures.
The secret to creating creatures from animals, plants or turning humans into monsters has been known for some time from serums and many have tried their hand at it. Some monsters may be intelligent, others may act by animal instinct depending on the maker’s design.
How To Make Monsters
Monsters are built just like any other invention by purchasing invention items with SP. Monsters can be made from scratch using invention items like Buy an Attribute. Another option could be if the monster comes from existing animals or even humans using the Trade an Attribute item.
One example of how a monster could work is the Subhumans in the example scenario The Diabolical Flying Machine.
Animals normally start out with intelligence attributes of -2 through -10 with trainable animals usually being higher on the scale than wild animals. In order for a monster to take verbal instruction, it must have an Intelligence of at least -5. For it to have the rudimentary ability to solve problems it must be at least a -2. To speak it must have at least a -1.
Frankenstein, Strange Case of Dr Jekyll and Mr Hyde and The Island of Doctor Moreau are examples of how a monster could be manufactured. The latter, The Island of Doctor Moreau is a good example of the Prolific Monsters story seed.