It's not that difficulty since there isn't much of a realistic limit to what monsters are capable of.
Herbivores, pelagi, carapaceon, neopteron, temnoceran and wyverns form a standard natural food chain and form an ecology. They all have strengths and abilities to aid them in survival, hunting or defending themselves.
Where the insane comes in, is elder dragons and a few exceptions to the wyvern rule (such as Akantor and Lucent Naruga)
You have Dalamadur and Raviente which are so big they literally change the shape of the earth by merely burrowing, creating rivers and the like wherever they go. In Raviente's case, he can destroy entire islands by doing that. Both of them are extremely old, reaching in the thousands of years
It also takes a 1000 years for a Gore to mold into a Shagaru, and wherever a Shagaru goes, the ecosystem will change so radically you can barely call it an ecology anymore.
Fatalis is a literal God. Fatalis, Crimson Fatalis and White Fatalis are the same creature at varying stages of their life. Apparently White Fatalis is ~10,000 years old.
You can think up the most inane shit and make it a viable creature in the monster hunter universe.