People say that the evolution of monsters in ancient times was different to that of monsters today. In the past, it is said that monsters exhibited various patterns of evolution depending on the characteristics of their local surroundings. This is a trait not found in monsters after that period.
-"Ancient Monsters" MR4
Monsters that appear the same sometimes take slightly different forms, depending on their area of origin. For example, take a Hornie, a monster born from the combination of a Suezo and Tiger. Depending on where the Saucer Stone was found and other differences leading up to its birth, the monster may be born with or without a horn. It is not known whether this is a factor determined by regional differences, or by a difference in the Saucer Stone itself.
-"Origins" MR4