Let’s keep things simple two rules.
- No giving sentience, This is a no brainer issue.
- Let’s keep it to beings under the Animilia kingdom. “mutated virus/bacteria” is a common trope.
To start:
Let’s modify ants to have lungs.
Most insects are constrained by the amount of oxygen they can acquire through their exoskeleton.
Imagine how big they can get if they didn’t have that constraint?
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.
The reason you don’t really see animals that can photosynthesis (other than microbes) is because you don’t actually get that much energy per unit of area. Think about how much area a cow has to graze vs the surface area of the cow itself. And much of the cow’s surface isn’t even facing the sun.
Not only that but that the energy from the nutrients generated in a plant is solely used for cell growth and maintenance. Even remotely suggesting a self-warming and extremely kinetic mammal can get energy solely from the sun is nuts.