Though he's made his live-action debut this month with Fantastic Beasts and Where to Find Them, Newt Scamander has always had a small presence in Harry Potter. He wrote the textbook on magical creatures eventually used by Harry himself, a book that would go on to be the first magical book available for Muggles to read (if you consider the book canon that is). But some astute fans have noticed that Newt was actually in one of the Harry Potter movies, if only for a second. He's never onscreen physically, but his name can clearly be made out on the Marauder's Map.
The above picture is specifically from Harry Potter and the Prisoner of Azkaban when Fred and George Weasley first show Harry the Marauder's Map. The map, which shows the current location of every person in Hogwarts, displays the name Newt Scamander, who is somewhere inside the castle. The names surrounding his are too difficult to make out, but that might not be important. This could be nothing more than a fun easter egg -- the real-life Fantastic Beasts book was published a few years before this movie came out -- but that hasn't stopped some fans from creating their own theories as to why Newt was visiting Hogwarts.
The theory that makes the most sense is that Newt was visiting Dumbledore. People who have seen Fantastic Beasts and Where to Find Them in theaters will know that Dumbledore was Newt's teacher at Hogwarts. When Newt was expelled from Hogwarts, Dumbledore strongly opposed and fought against the decision, suggesting that there's some sort of kinship between them. Dumbledore also writes the forward to Newt's book, so maybe Newt's visit was in regard to that.
Another theory only gets stronger once you consider one of the key events of Prisoner of Azkaban: the execution of Buckbeak the hippogriff. It doesn't take a stretch of the imagination to think that Newt would take a special interest in Buckbeak. Newt is a lover of all animals, and Buckbeak's unfair sentence to death would draw his attention. This only makes more sense considering that Newt's mother was a breeder of hippogriffs; Newt had an early love of magical creatures. He could have been there to try and stop the execution, but he was unsuccessful.
Newt's name could be nothing more than a harmless easter egg, but it's always fun to speculate these things, even if it's only head-canon. Though he's not physically in any of the Harry Potter films, you can watch Newt Scamander take the lead in Fantastic Beasts and Where to Find Them, which is currently in theaters right now.