The world of Melody Box, spoilers included.

[Robin Ryuu]

The people in the story can create chimeras (a new creature created from combining two or more preexisting creatures) but, no the ability to create completely new creatures went down with the main connection (the tower) to the melody box.

Also you need to know about the objects you are trying to alter, the more you know the easier it is. In this case it is kinda like the true name system except knowing somethings true name is an unreachable concept like perfect.

There is an exception to this rule. If like you said someone doesn't know they have magic and somehow uses it while thinking of something they could potentially create a "ghost". Ghosts in this case are energy masses that take the form of the thought. In this world the laws of matter and energy exist, so the energy needed to create the ghosts has to come from somewhere and for someone who doesn't know about this they will most likely draw energy from themselves or from those around them which can kill them. Because of this magicians need to learn to control their power quickly. Also ghosts do not need to be in the form of something living or formerly living they can be an item or a place or even something that doesn't exist. And most ghosts are unstable and will most likely fizzle out after their energy supply has been used up, there are exceptions.