A shifter, or shapeshifter, is a person who can tranform their shape into one of four forms: dog, fox, parrot or raven, with the fox being the most rare. It's said that for nearly a thousand years - closer to the world's beginning - there was a fifth form: cat, but the form has largely been lost, and the only cats remaining on Umayma are the beasts of burden which pull carts and rickshaws.
The reason for these particular forms and no others remains a mystery. The long fall from the moons to Umayma - after the calculated colonization by the First Families - is believed to have been unplanned. The only animals surviving the rushed exit from the moons were cats, dogs, foxes, parrots, and ravens.
It's known that shape shifting - unlike the talent of magicians - was not an ability brought down with the First Families from the moons. The ability of colonists to shift appeared within two generations of the mass colonization. Some speculate that it's simply another uncontrolled mutation created by magicians. However, because there tend to be more shapeshifters among groups settled on some areas of Umayma than others, it's thought it may be genetic.
Most shifters exhibit their ability first in puberty, though there may be early signs such as severe allergies, including such benign symptoms as watering eyes and sinus infections to skin rashes, hives, and benign growths.
Shifting is a messy and uncomfortable affair. After shifting from animal to human form, most shifters have an overpowering need for protein - a desire that can often bankrupt a household on Umayma, which receives much of its protein in the form of various types of bugs or the more expensive and rare dog types that are bred for eating in select countries.
Magicians can often sense when a person is a shapeshifter, as the air often seems to "prickle and bend" around them. This has led to the secular belief that shifters are, in fact, the product of some old magicians' botched concoction, and may bend and tear reality itself, displacing the mass of their human forms and storing it in some other space or reality while in animal form. Some have even speculated that shifters are able to open gateways between the moons (where their mass is stored or exchanged) and the surface of Umayma, gateways which operate in much the same was as the magicians' gateways which link disparate regions of Nasheen.
However, like the magicians' gateways, whatever technology or knowledge existed to create or tailor these phenomenon has been lost. It is believed that whatever power the shifters have, it has been mutated or corrupted from its original purpose.
From God's War:
The outrider stood. She looked uneasy, like a cornered animal—a dog-shifter in form, or maybe some scraggly adolescent sand cat. He might have guessed her for a shifter if he had seen only an image or picture of her, but in person he was able to see clearly that she was not. The air did not prickle and bend around her as it did a shifter. She was just some kid, some standard—just another part of the world. ...
“Something else she said, though—wanted to know if I’d ever fought a shifter. I told her we don’t allow shifters in a ring, and she said why not? I told her it was ’cause they had an unfair advantage. She thought that was stupid, you know, since we’d fight magicians and all. I had to tell her being a magician just means you’re using bugs. Magicians can stop using bugs. They can even drug up the bad ones to keep them from using bugs. But shifters, it’s in their blood. They’re half us, half something else. Told her the First Families used to call them angels. She was real interested in that.” ...
“Naw, she thought they were just like magicians. Called on certain bugs or something to change them up. I told her no, they were something else, something that got fucked up at the beginning of the world. Told her shifters live half in this world, half in the afterlife. Angels.”