Dragon and Eldritch Horror
Breath weapon causes insanity (save for half damage).
Unless you mean real things...
Mantis Shrimp and Megalodon
Size of a bus, hits like a bus moving fast enough to break the sound barrier.
Or specifically extant creatures...
Manburg Virus and HIV
Basically, if you even look a man of color, you're going to die.
Or only living creatures whos life status isn't a controversy if it's a thing at all...
Fire ant and Bald Eagle
That bullshit comicbook thing about porportionate strength, a bite that ignites you with invisible flames, flight speed nearly reaching 100 mph, their ability to build nests that support hundreds of them, and whenever they attack that song from Team America (you know which one) blasts in your ears making it hard to communicate with your allies.