"The Inner Eye can be a burden, you know..."
-- Sybill Trelawney (PA16)
A Seer is a witch or wizard with the Inner Eye, that is, the ability to see into the future.
Cassandra Trelawney was a famous, talented Seer (FW).
Her great-great-granddaughter Sybill Trelawney also possesses the Inner Eye, but she has only truly used it on two occasions (PA22).
Other seers include the Ancient Greek rivals Mopsos and Calchas; Cassandra Vablatsky, the author of Unfogging the Future; and infamous dark wizard Gellert Grindelwald. (FW, JKR:Tw)
While unnamed, more seers exist such as Naming Seers (Pm), and whoever gave all of the prophecies in the Department of Mysteries. (We briefly hear snippets of two of them in OP35, one from an "old, bearded man" and one from a "young woman".)
In the original drafts for the books, there was a blind character named Professor Mopsus who was skilled at Divination. Parts of him can be found in Trelawney, Mad-Eye Moody, and (Ancient Greek) Mopsus.
Richard Wheatley for the RNIB: - Blind children everywhere are delighted that they can read this book at the same time as sighted people, would you ever include a blind character in one of your Harry Potter books?
JK Rowling: Funny you should say that because at one point there was a blind character who went by the name of Mopsus, and I will let you look him up because there is a mythological connection there, but he sort of that was a very early character and he had the power of second sight, in other words he was a bit like Professor Trelawney, he was a very, very early character, this was when I was drafting Philosopher's Stone, the reason I cut him was he was too good. As the story evolved, if there was somebody who really could do divination at the time that Harry was alive, it greatly diminished the drama of the story because someone out there knew what was going to happen.
So that is why Mopsus went and I have never really replaced him, although I suppose Mad-Eye Moody, had some of Mopsus' characterisation. He has one magical eye because he lost an eye in a fight with a Death Eater, so good question.
(Edinburgh "cub reporter" press conference, ITV, 16 July 2005)
From the Web
Has anyone other than Sybill Trelawney made a real prediction/prophecy in HP world? from Scifi StackExchange