Let’s not leave the plant Kingdom out of the fun.
Gigantic Venus Fly Traps!
Would keep the teeming hordes thinned out at some of our crowded parks and recreation areas.
Thank you!
Folks, that was a lot more interesting / entertaining than I expected. Do click the link.
Hell, in Oz even the cute and pretty creatures are dangerously fierce.
Madagascar had the now-extinct Giant Fossa, basically a six-foot long man-eating weasel.
Could it dance? (Stupid joke, I know.)
Or incredibly cuddly. Have you seen a Maine Coon Cat?
That’s why it has 12 legs:
It’s also largely hollow, partly negating the cube-square law. Hayao Miyazaki was clearly a student of J. B. S. Haldane.
That thing in Sam Stone’s pic needs to be killed with fire. Evil personified. Even of there is forced perspective going on and the woman is teeny.
Giant otters look big enough to be dangerous (6 feet long). They’re social, hunting in groups, sometimes taking on caimans and anacondas. Groups of them can drive off jaguars.
You wouldn’t have to enlarge them much to make them nightmare fuel.
And then of course there are the giant apes. Regular chimps are dangerous enough.
I thought of a chihuahua the size of an alligator. It would be Cocaine Bear level scary.
And don’t get me started on the 300 foot tall walking lizards. Those would definitely be hazardous.
Sharks.
And going the other direction The Incredible Shrinking Man, or Antman.
Little dogs.
If you want dangerous animals, don’t make them bigger - make them more venomous. How about mosquitos with botox in their saliva?
Piranha dogs?
Dragonflies are supposed to be the most efficient hunters, catching up to 95% of their prey. In the Permian (due to the higher concentration of oxygen already mentioned), there was a dragonfly with a 75cm wingspan.
That just confirms my long-held belief that all Australian wildlife is out to kill you! 
Between giant deadly spiders, snakes, crocodiles, jellyfish, and even house-eating cockatoos, enlargement is not necessary for these creatures to be fearsome and deadly!
And of course, Australia is one of the very few places in the world to which the cassowary is native (along with Papua New Guinea). The cassowary is about the closest thing we have to a modern-day dinosaur, and indeed is said to be descended from the velociraptor. This guy (in the US) had a cassowary as a pet and it nearly killed him:
Judging from the way Tootsie has swatted my hand when I stop brushing her, before she’s finished being brushed, I have to agree.
Obligatory photo of Tootsie:
@Sam_Stone Photoshopped, or corroborated?
That makes me think of the sf novel “In the Courts of the Crimson Kings” by S.M. Stirling. Set on a counterfactual Mars that’s habitable, the protagonist has to pass through a lethally biohazardous area, where swarms of communal carnivorous mice are the least dangerous lifeform in that area.