And here is Bookworm Gal's two cents on "Danny phantom"
Ghosts weren't necessarily the souls of the dead, though the loss of human life could certainly spark their creation. One definition used to describe them was an "odd manifestation of ectoplasmic energy and post-human consciousness," but that was only one form and one way that they could come into existence. There was more variety to the inhabitants of the Ghost Zone than that.
Some ghosts were spontaneous creatures that emerged from the ectoplasm or were given form by other specters. Those tended to be the least human-like and often the least intelligent, though there were exceptions. Skulker, after all, was not the spirit of a living hunter, but a small green ghost who developed a battle suit to appear more humanoid and yet was as intelligent as any other specter. Given time, they could become smarter and grow more complex. Many, though, would remain as creatures that obeyed orders or acted on instinct rather than having a developed personality and mind.
Other times, a death of a particularly strong-willed person or one with strong emotions connected to the event would leave an echo of that person to form in the Ghost Zone, providing a template for their appearance, personality, and even occasionally memories. Sydney Poindexter, for example, retained almost every memory of the human who once suffered so much bullying during his life and even managed to recreate the familiar school and inhabitants due to the strong emotions surrounding his demise. These ghosts were the closest to what was considered a "traditional" ghost. They weren't quite the souls of the deceased, but they were close enough for most.
The third type of ghosts was also spontaneous creatures that form from the ectoplasm of the Ghost Zone, but are the result of human belief rather than by mere chance. Belief, like strong emotions and will can leave echoes behind that are felt though the same bond that connects the Human Realm and Ghost Zone. Myths, legends, and folk stories that are told enough generate belief and can cause ghosts to take those forms. This can range from the creation of werewolf ghosts (Wulf) to Pandora of Greek mythology to anthromorphic personifications of complex subjects such as time (Clockwork), nature (Undergrowth), and sleep (Nocturne).