Cows now have tetrodotoxin like Fugu fish. Incorrectly prepared cows now have the chance to kill you.

A breed of 100+ pound Chihuahua with the same temperament as the original.

A new species of bear develops the temperament and train-ability of a domesticated dog.

People love the domesticated bears. They’re so big and cuddly. The bears quickly become the most desired pets in the world.

Bear breeding farms sprout up all around the world where the bears are subjected to awful conditions. There’s a general outcry against this but people keep buying the bears and the cruelty doesn’t stop.

Many of the bears are abandoned by their owners when they become adults and the owner realizes how expensive feeding a fully-grown bear is. And that’s before we get into the issue of the piles of poop the animals leave behind!

Although feral bears are smaller in size than their wild cousins, their sheer numbers makes their dominance in the wilderness inevitable. Habitats are destroyed and wild bears all become further endangered as the feral bears encroach on their territory and compete for food.

Animal control in every part of the world needs its budget doubled or tripled to deal with the problem, but many governments are slow to act on the issue.

Feral bears without fear of humans become common sights in small towns and the outskirts of cities, where they reap unmitigated havoc and destruction.

The governments of the world finally respond as the feral bear populations make their way into wealthier areas, but their responses tend to punish responsible bear owners as much or more than they actually address the bear-related problems.

In America, the Supreme Court rules that the right to “bear arms” includes the right to own a bear, so the few places that manage to have sensible restrictions on ownership are forced to drop them.

Both male and female mosquitos now drink blood and they spit some of the last person’s blood in you when they drink, causing them to spread blood borne diseases

House flies start producing potent methamphetamine in their wings, which means the whole world has uncontrollable access to it.

Give any animal the ability to photosynthesize. Now animals are of course complex creatures that need a variety of nutrients to function properly, and the number of chemicals we’ve seen be able to be photosynthesized is low, but imagine only needing to nude sunbathe for like a half hour to get enough sugars to fulfill your caloric intake, including the less efficient carb->protein/carb->fat conversion and pop a pill for some vitamins. Imagine if all animals had that as a baseline and just needed to hunt/forage for nutrients and vitamins to support auxillary functions.

Overpopulation would be nearly impossible despite massive population booms, with the only real limitation being physical space and the social dynamics of any given species or interspecies interactions.

Mhh, insects in general IIRC are also constrained by the fact they have exoskeletons: i.e. their internal organs are just in a bag. So they can’t grow too much or they just squish I think.

Imagine them being able to become vertebrates. You can have insects the size of cats I think before the osmosis breathing tubes they currently have are not a thing. Lungs would be the next thing I guess.

Disclaimer: I’m not a biologist and just make stuff up from what I remember from high school biology, and it was last millennium!

Not very up on biology, so not sure if this would even be a thing, but I would say some kind of internal structure like plants allowing animals to overcome the square-cube law

Ant phagogenesis

Give human level intelligence to gorillas, this is worse then just human level chimpanzees in the fact gorilla’s are beefy and could rip the average human in to pieces. With human level intelligence they would be a force to me reckoned with

Some common form of bacteria modified to break down plastics rapidly the way they do organic matter. Youd see buildings falling apart, packaging need to be redesigned for everything, electronics rotting, etc.