Improved monsters without the “In the Wake of Gods” specifications.

Without enabling any options, it's impossible, but they can be hired in mercenary camps, obtained through diplomacy, or, if you've captured a dwelling and you're either playing as Dungeon and have a summoning portal built, or you're playing as a faction to which the 8th-level creature belongs and have a city with an upgraded 7th-level dwelling. And the 8th-level creature's recruitment rate doesn't depend on other buildings or the 7th-level creature's recruitment rate; you can safely hire both types of creatures.