Phantoms are mentioned several times in the Harry Potter series, but it is unknown whether or not the term is used to discribe an actual magical species, or any being who is skilled in stealth.
Phantoms are first mentioned in the Chamber of Secrets when Harry wonders if the owner of the strange voice he's hearing is a "phantom" that can move around unseen. It is unknown whether he is reffering to an actual magic species, or any kind of creature with the ability to move around unseen.
Later in the Chamber of Secrets, when Harry is inside the Riddle Diary and realizes that no one can see him, he thinks that he is little more than a phantom. The way the words are fraised here, it is more likely that Harry is reffering to an actual type of magic creature.
ABOUT PHANTOMS(FYI, I accidentily erased the Section Headline that was here, and have no idea how to make it reapeer in big print over an underline. If someone would please fix that for me.)
If Phantoms actualy exist, they are a race of spirits closley akin to ghosts and poltergeists. Hence the name, they are are highly stealthy, and can probably (a hint of of their relation to poltergeists). Due to the initialy negative reatction the name gets, phantoms are most likley crafty and malevolent. Phantoms are most likely on of the Wizarding World's many species of dark creatures.