You could just give them a description of what they see, and how the creatures behave. I always allow my PCs to roll a knowledge check (in my game, all knowledge checks can be performed by anyone. There's always a chance, although sometimes very slim, that someone knows something about anything. I determined the DC depending on how likely I think their character would know something.)

I try to keep my players from remembering the stats on some monster by tweaking everything. In the real world, there are dozen, hundreds of species of a same animal or plant. Creatures might get more aggressive during mating season, to protect their young, or perhaps they have a bad tooth, all of which might drive a neutral creature to attack with surprising viciousness. Creatures can age, get sick, or become crippled after a fearsome battle, so a formidable opponent might freak out your players an yet be a challenging foe, unexpected, worrisome, yet beatable.

For example, it's plausible that individuals of a certain species of ankhegs are smaller than their common counterparts on which that stats are based. Maybe they are more agile and run faster, perhaps they don't have an acid attack, but that they have a bite with a paralyzing poison, maybe their exoskeleton is a dull grey, making them look like any other rock in their native habitat, the foothill of a great mountain range. Maybe another cousin of them, living in the rain forests, digs pit traps at the bottom of muddy slopes, the bottom of which is a teeming blend of watery mud, potent acid, and hundreds of hungry baby little ankhegs.

Perhaps they encounter a motherfucking ancient black dragon, but it is so old that it is half senile, partially blind, one of his wings was once injured and healed wrong so he has a very poor flight, he has a bad cough which causes his breath attack to be short range, as well as slightly erratic, pathetically dripping on his chin and splattering unpredictably as his aim sucks, and his natural armor, his large, dull black scales shows numerous patches of encrusted sores. Still dangerous as fuck, but perhaps not deadly with some luck and strategy.

So yeah, first I tweak something that they haven't exactly met before. Second, I tell them what they see / hear / smell. Third, I make them roll a knowledge check. DC and information (quality, quantity and accuracy) depend on their class, background, how prevalent the monster is in the region/world, how accessible the creature is to study, and additional factors (sickness, injuries, magic, subspecies particularities, etc.)