Monsters are an incredibly important and valuable part of the RPGverse ecosystem.
Any RPG needs something to slaughter in the hopes of gaining EXP and leveling up. People were a little rough for this setting, so good old monsters that you can't feel sorry for were the obvious choice. There are a lot of monsters, ranging from tiny to megalithic, rock-dumb to sentient etc. in RPGverse. Generally, the bigger and smarter the monster, the more EXP they will give.
As a rough guide regardless of the region:
Common monsters include feral livestock, fantasy wolf/bird/bear equivalents, and Haror - a catch-all term for aggressive plant matter (think angry mushrooms). These are the sorts of creatures that an 'ordinary' person would encounter during their life and should be able to defeat.
Uncommon monsters are more difficult to stumble across and beat. These begin to venture into things with magical ability. Felengar/thundercats (pictured on the previous page), flans, goblin-esque humanoids, mimics etc. fall under this category.
Rare monsters have a purpose to exist. They aren't the kind of creature to wander into a farmer's field because they want a bite to eat - they are often sentient, and have some grasp of language and organisation. Most dragons, vesella/mermaids (above)/lamia/naga, and elementals are considered rare. When talking plain rarity, as in lack of abundance, there are quite a few animalistic, non-sentient monsters which have low numbers and/or produce rare materials.
Super Special Rare monsters are created by the Patrons as needed - notably the megafauna. They're made with the sole purpose of stirring up the inhabitants of RPGverse to compete, either through a rampaging colossus or a huge 'raid boss' with valuable loot associated with it. Not all megafauna are necessarily destructive or malevolent beings. It just depends on the time and circumstances of their creation.
All monsters respawn, either through breeding or by manual replacement by the Patron of Monsters if their numbers get too low. Of course, the Patrons make sure to balance resources in RPGverse and may cause dips or spikes in monster populations in order to spice things up.