Asklemmy

Octopodes no longer die when they give birth, meaning they can teach their young and form societies.

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.

Ok, take this with a grain of salt because I read about it ages ago in a dubious pop-sci book and my memory is shaky. One time, they tried to gene edit yeast to be able to survive much higher alcohol concentrations. There's lots of good reasons to want to do this... Beer/wine is just about the strongest beverage you can make without distillation of some kind because the yeast dies. Making way higher ethanol yields just from fermentation makes biofuel way more viable. Stuff like that.

EXCEPT... It nearly escaped, and was able to survive on it's own. Yeast is very ubiquitous in nature, so a wild yeast that can tolerate massive ethanol concentrations could conceivably have altered life on earth as we know it.

A cursory internet search isn't turning up anything about this, but I'm pretty sure I read it in the book Everything Is Going to Kill Everybody, if anyone wants to look harder than I did.

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

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.

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.

In a push to deal with micro plastics, bacteria are developed to be able to break down plastic. Eventually, it gets into the plastic installed for a purpose and starts breaking that down.

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!

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

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

All life with gills learn to breathe air without water, and suddenly all marine life is competing with us for land space

Amaranth that can grow on sand and be watered with seawater

Edit: ops, only animals? Make bees able to break down any kind of sugar into honey

A malicious gene drive. Basically, use molecular tools to ensure that a gene is always passed down from a parent to its progeny, regardless of the other parent’s genetic makeup.

Many choices available: propagate resistance to a pesticide for mosquitos, guarantee Huntington’s disease in a family, or crash a population of beneficial species by reducing fertility, to name a few.

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