This is more like an observation than an actual theory
All of the Mythical monsters were made by the Colossals, except one: Anglow.
Anglow is from another planet, and was later classified as a Mythical by the other beings of the monster world.
The Colossals created the Mythicals with certain elements or islands (Continent's Elemental Lands at the time) in mind, being Plant, Cold, Air, Water, Earth, and Fire. All 6 natural elements.
Oasis is an artificial construct that was made many many eons after the Dawn of Fire.
I believe Cherubble is the true Water Mythical, but as shown in bios, it fled the Colossals and went to live its own life. After the newly made Cherubble fled, the Anglows soon arrived. The Colossals, seeing a new aquatic species arrive on the planet in the golden age of the Mythicals, were pleased, and knowingly accepted the new Mythical monster of Water into their world.
Yeah that's it
I know it's pretty obvious if you think about it for any more than a minute but I still wanted to talk about it