Descriptions will be fairly descriptive, since there's no comparison to existing creatures. We all know bears are dangerous and chickens aren't (debatable, but still). Voracious Cave Crawlers and Elk Birds are two things that occur to me have no comparison, and thus their danger level is hard to judge. But, given the hardiness of the player base, who turns down Giants with two copper-clad soldiers and throws titans into gladiator pits, a creature like a blind cave ogre will be listed as "Dangerous to Civilians" while a proper-sized demon, and there will be HFS levels of powerful creatures, will be "Extremely Dangerous". Let's face it, we've all faced a demon made of steam that poofs, or even some sort of toad demon that isn't that tough. I'm talking about bronze-skinned creatures with stone organs and NO_FEAR NO_EXERT. Wildlife isn't a good challenge to players after a bit of playing, and some of these creatures will try and revive that challenge. Others will be rather placid or unicorn-like. You'll have to pay attention to their color descriptions though. If its description says "It is very dangerous" it means it's large and aggressive, and if it has a color modifier for "Its claws are teal and hard as iron" then it's best to face it cautiously.
To summarize, there will be good descriptions, and it won't be as vague as "It is dangerous". If you pay attention you can figure out "It is enormous" + "It is aggressive" + "It has large claws" = Fun!