Races

Alicorns
Very rare, only one brain known to exist in the region
Brains are very large, bulge out of their skulls, almost mane-like

Griffons
Griffons appear to behave normally without any brains, after a brief adjustment period.
Derp and drool are still present while brainless, as is loss of intellect.
Adjustment period is anywhere from one day to a week
Can function at a baseline intelligence without brains
INT stat capped at 3 until brain is replaced
Cannot read or write, but can speak normally

Robopony
Roboponies are plentiful in this part of the Wasteland, always with a pastel-colored brain nestled in their clear domes. There's a mix of tracked and legged bots, but all have two tendril-like manipulator arms tipped with claws to grab objects in lieu of telekinesis.

Goo Sacs
A parasitic creature made of goo, it paralyzes its victim with a tendril slid up their behind as a direct means of attaching to and coating the spine. It slurps their brain out the same way and integrates the organ into itself, using it to think with and gaining access to all of its memories. It attaches itself to the victim's genitals and absorbs them, replacing them and taking their form as a kind of disguise, then uses the debrained body to carry itself to more brains, growing its intellect and expanding the physical brain nestled inside itself.
As the creature grows older and eats more brains, it gains an incredible intellect, despite appearances. Being solitary creatures, it does not often travel in groups and only occasionally infects another body. If it encounters a body that is objectively better than the current one it is attached to, it can migrate to the new body and leave the old one behind.

Goo Coats
A parasitic and slightly magical goo-based creature created by random chance from interactions of Taint and Balefire Radiation. This amorphous blob attaches itself to the brain of its victim, completely overwhelming their nervous system and rewriting the neurons inside to adapt it to their needs. In other words, it brainwashes its victim and then takes complete control of their mind, effectively turning it into theirs. These Coats form a sort of pseudo-hivemind capable of sharing thoughts and senses between members across distances of up to 1 mile or 1.5 km and spread by direct contact with uninfected neural tissue. This can take the form of a tendril through the ear or just slapping a hoof-full of goop onto the brain directly. Once a brain has been converted, it cannot be cured and will continue to produce goo even if completely washed off (at which point the brain stops thinking until the layer is regenerated). Coats take on personality traits of the brains they infect, thus making each goo an individual while still retaining some base level traits of their 'family.' Hiveminds are formed based on lineage and not all Coats are in the same hivemind nor can share thoughts across different 'families.' Each 'family' is identifiable by the shade of goo they produce.
The only involuntary physical difference after infection is a change of the victim's irises to match that of the goo's color and a slight tinting of the brainmatter. For comfort, the goo often cuts and detaches the skullcap of the host to give itself more room to grow. If necessary it can reattach the cap to disguise itself, but the increased pressure may hinder the brain's performance.
Due to the forceful nature of the rewriting process, the brain is slightly damaged and thus suffers a -2 INT penalty after conversion. This cannot make brains to into negative numbers and bottoms out at 1.

Diamond Cyberdogs
The Diamond Cyberdogs were created pre-war as an attempt to create the ultimate soldier. With durable robotic limbs, neurochips in their skulls giving them the precision of a computer, and a pony’s brain linked to them to allow for creative and abstract thinking, they were, along with the Robopony, the pinnacle of cyborg technology. 
After the war, the Diamond Cyberdogs were freed of their masters’ control, and soon became focused on not only growing their numbers, but gaining access to suitable brains. A lack of brain preservation technology meant the pony brains within their heads would deteriorate after roughly half a century… and the difficulty of preserving their brains would lead to them developing a culture that was rather… cavalier with the abuse and damaging of the minds within their skulls. As a result, they raid far and wide to acquire said brains… and as a side benefit, replace the brains they take with a robot’s processor to create a perfectly loyal slave.
They live out the first few years of their lives as normal Diamond Dog pups, but when they come of age they are cybernetically modified, and sent out into the world to acquire a suitable brain to replace their original. Once they have done so, their original brain is ripped out, neurochips containing copies of their memories are installed, and their stolen brain is implanted- at which point, the new Cyberdog smugly throws away or sells their helpless original brain. Their original brains understandably end up bitter about this, because they are lied to from birth about how their consciousness is tied to their brains.