Frame Challenge: You don't want chemicals, you want engineered viruses.
Bioweapons Run Rampant
The post-apocalypse tropes of "giant animals" was made popular in the 60s, but isn't remotely realistic. Given current global conditions, bigger and fiercer isn't a particularly good survival strategy; smaller and more fecund is. So how do you make your animals more threatening?
As part of the apocalyptic war that largely wiped out humans, one particularly hackneyed plan modified rabies and made it airborne, further modifying the pathogen to delay onset and increase aggression towards larger animals (or, via some magical science, humans in particular). The resultant swarms of animals of all sizes attacking humans spread quickly beyond the borders of the country upon whom it was deployed, but after humans vanished from the planet, became largely latent aside from driving all large primates to extinction.
As humans reemerge, a pathogen that occasionally drove animals into suicidal rage but mostly remained in faunal reservoirs goes active again in a big way. Animals routinely start frothing at the mouth and seeking out humans to attack, and a bite is almost inevitably fatal unless treated immediately.
The animal populations on the whole appear largely unchanged over the thousand intervening years - they'd all be recognizable to a contemporary human - but this nasty little virus makes every mammal bigger than a squirrel into a potential death sentence.