I wrote a bit about Peeves here, but will copy it into this post.
Poltergeists do not spawn from the spirit of a person who has died, like a traditional ghost; a poltergeist is an evil or malevolent energy that forms from negative or tumultuous feelings or vibes from a living person. Peeves is a bit of an exception, for he is corporeal. Traditionally, poltergeists are not (they knock on things and make scary noises, and seem to exhibit the most activity in relation to one specific person whose energy the poltergeist is feeding off). Poltergeists can be quite damaging to property, though, as Peeves demonstrates throughout the series. Anyhow, Peeves could not have been a spirit that had once been a person.
Here's a little more information about Peeves from a question I answered here:
ES: Why does Dumbledore allow Peeves to stay in the castle?
JKR: Can't get him out.
ES: He's Dumbledore: he can do anything!
JKR: No, no, no, no, no. Peeves is like dry rot. You can try and eradicate it. It comes with the building. You're stuck. If you've got Peeves, you're stuck.
ES: But Peeves answers to Dumbledore -
JKR: Yeah. I see Peeves as like a severe plumbing problem in a very old building, and Dumbledore is slightly better with the spanner¹ than most people, so he can maybe make it function better for a few weeks. Then it's going to start leaking again. Would you want Peeves gone, honestly?
MA: If I was Harry I might, but as a reader I enjoy him. I enjoyed him most when he started obeying Fred and George at the end of Book 5.
JKR: Yeah, that was fun. I enjoyed that. That was satisfying. [Laughter]
MUGGLENET - J.K. ROWLING INTERVIEW - EMERSON SPARTZ and MELISSA ANELLI - 7.16.05
What isn't clear is how long Peeves has been at Hogwarts. His bowtie and bowler hat indicate he might be from a more modern time than from 1000 years ago; otherwise, I imagine he would be wearing a ruff and a jester hat or somesuch.