Most of the world's population are part animal, though 1 out of 10 people are born without any animal parts.
The average person is %20 animal, %80 human. A part-bird person, for example; would only have wings and a tail, perhaps a few feathers on their body if they’re lucky. With some training done at a young age, people can increase the number of animal parts they have. Mostly, people with predatory animal halves do this to grow fangs or claws if they have an interest in self-defense or joining the army, etc. Though it is forbidden in most countries, sometimes people would be encouraged to do that to serve their country in times of need. There are only myths of people who were able to transform entirely. It is possible but extremely difficult, it takes a lifetime to transform fully and it requires unhealthy amounts of training. Many people have passed before they could completely transform. The most known myth about this is Split Idol. (Which is a big motivation for one of the characters, Samira.)
Animal halves would also determine one's job. Of course, they’re allowed to chose what they want to do, but if that fails they’ll always have a backup plan. For example, if you’re part-pigeon, you could easily become a postman of some sort. Part-owls would be librarians, generic stuff...
...This is not to say people without animal parts don’t have a part in society. They get to work of course, though there are ways for them to earn an animal half. The most known way is to enchant an object with an animal soul and carry it around. The object of choice would usually be a solid gem. After an animal's spirit has bonded and fused with a person, the object could not be used by anyone else as long as the first owner is alive. The animal in question is called that person's "familiar" after bonding.
(This process can not be done with human spirits.)
This doesn’t work with every animal though. The animal's spirit is more likely to “bond” with the owner of the object if the person had a positive connection with it before it passed away. And some “mythical animals” like dragons have a mind of their own -Most species’ brains are not as advanced as humans’ but they can still decide for themselves- they might choose not to serve the owner.
As we call them: “Mythical animals”, have gone extinct years ago, and finding an object that’s been enchanted with such creatures’ souls would be extremely hard, dangerous, and costly. What I mean by dangerous is, these stones would make the owner quite powerful –especially ones with dragon souls- and others may want a piece of that. They could attempt to hurt the owner to get to be able to use the stones. It would be tough if the owner catches up to their plan, they would be able to stop them –maybe by threatening them since they’re most likely stronger than the other person- it's better to be cautious those sorts of people.
Someone can’t be fused with two animals if both creatures are spirits that bonded with the owner after birth because the ghosts most likely wouldn’t want to cooperate, however, someone who was already born with an animal part can get themselves another animal part by using an enchanted object as the animal part they were born with isn’t connected to a spirit. This is not allowed in most nations, only leaders get to do it if necessary.
Though most of these animal parts can not be “turned off”, and if they can, only requires the owner to think about it, some need a trigger to be visible. The owner needs to pick a specific part of their body and scar it in a way the creature would understand its presence is required; this is called a "Calling Scar", they need to be in a specific shape that's determined by researching the spirit animal in question. They also need to be deep enough to never disappear. These hot-spots vary depending on the species of the animal. This is most common with hostile animals and dragons.
Because being part dragon can make one very powerful, humans have driven them all to extinction for their purposes. As mentioned earlier, most dragons have a mind of their own. They can decide for themselves and some can even speak. Though their grammar would be terrible. Only elderly "alpha" dragons can talk normally.