In IR, monsters grow in strength via three ways; either by levels, ranks or both. On a general view, all monsters grew by levels but there was a cap on their growth based on their potential. Using the [Lorlian Sea-Blue Crocodile] for example, its average level cap at maturity was between level 17-18; very few of the species made it to level 19 and none could reach level 10.
Although monsters generally grew in level, they also generally grew in rank with their average cap set to Elite with very few reaching Rare-Elite. Using the same example as above, most [Lorlian Sea-Blue Crocodiles] were either Common, Elites and very few at Rare-Elite rank.