There are many, many different species in IDET, but because there are so many different species, there are creatures that cannot be identified by species, and we call them creatures.
In other words, they're creatures of a different species, but their exact species is unknown.
Therefore, the livings called 'creatures' tend to vary greatly in their characteristics from individual to individual.
They are also the most common creatures in IDET.
Sometimes creatures are later revealed to be of a species, in which case they are no longer monsters and take on the name of their known species. (Examples of such races include vampires, witches, and dragons).