The Breakdown of Monsters, Creatures, and More.

In the eyes of the people of Aloran, there are only four main categories of life, excluding plants.
The first is the Races. These are your humans, elves, Minotaurs, Halflings and more. The Races are basically sentient humanoids that are capable of building complex societies and developing relations with other races.
After that, we have the Monstrous Races. These are humanoids that are sentient but either do not or cannot build complex societies or they do not develop relations with other races but rather see them like cattle or lesser. Kou-Toa, Feral Goblins, Gnolls, Chittor, Illithid, Fungun, giants and more comprise this category. Not all Monstrous Races are hostile towards the Races, just most.
The next category is Creatures. These are generally non-sentient, non-humanoid beings. The main trait that sets these apart from the next category is that Creatures, while can be dangerous for the Races, are far less harmful and often times they can be tamed or coexist with the Races or Monstrous Races. These are your cows, Beehemoths, Wyvern, tigers, Sky Whales, bunnies, etc.
Lastly, we have Monsters. Generally, these are non-sentient, nonhumanoid beings that pose a great danger to the Races and sometimes the Monstrous Races. Examples of these are Aboleth, Rust Monsters, Deep Crow, Beholders, Hydra and more. The danger they pose to the Races is the defining trait of all Monsters.
It is worth mentioning that not all beings have all of the traits of their category. A prime example of this is Dragons. Despite being highly intelligent and capable of building societies if they so wished, their non-humanoid form and the danger they can present to the Races have placed them within the Monster Category. Another good example is Grungs. They build complex societies and sometimes develop relations to their neighbors but their general view and behavior towards the Races have caused them to be categorized as a Monstrous Race. 
Another thing worth noting is that closely related beings are not always in the same category. Continuing with the Dragon example, True Dragons are considered Monsters, regardless of color, but Wyvern and Drake are considered Creatures. The three of them all belong in the Dragon Family but are placed in different categories.
These categories only truly apply to a species as a whole. There can be individuals within that species that display traits of a different category. This could be a Gnoll who has decided to become peaceful and lives in a human city, a Sky Whale could harbor enough animosity towards surface beings that it tries to kill all on sight or even a Beholder that behaves more like an elf or human than other beholders. Sometimes there are enough of these contradictory individuals to warrant separate classifications for both. This happened to Goblins. Enough Goblins began exhibiting behavior more like a Race than a Monstrous Race so Goblins were divided into two groups, Goblins who became classified as one of the Races and Feral Goblins who remained classified as a Monstrous Race. While both are physically similar their intelligence and behavior greatly differ.