My guess, based on past sets, is that at least half the cards will be on-theme. Look at the other sets with strong themes: Seaside, Alchemy, Dark Ages, Guilds, Nocturne and the proportions of those that related to the theme.
(Good luck figuring out which cards match the theme of "hinterlands". (-8 )
As for making an animal kingdom, that's already just-about possible: Familiar, Trusty Steed, Rats, Magpie, Wolf Den, Faithful Hound, Goat, Bat, Locusts. You might need to stretch a point and include animal-related things like Wolf Den, Werewolf, Menagerie, Stables, Hunting Party, Hunting Grounds, Wild Hunt, Horse Traders, etc.