Why was Newt Scamander's name on the Marauder's Map when Fred and George gave it to Harry?

Fred and George gave the map to Harry because his needs were greater than theirs. When he was just closing the map (mischief managed), Newt Scamander's name was there.

According to the timelines, Newt was expelled from Hogwarts long before Harry was even born, wasn't he?

Harry Potter: The Prisoner of Azkaban.

Answer

There are few theories for the visit and none of these are mine.

Theory 1: Visiting Dumbledore

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 [sic] to Newt's book, so maybe Newt's visit was in regard to that.

Theory 2: Saving Buckbeak

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.

Theory 3: Just an Easter Egg (Most Probably True)

@RichS, Thanks for sharing this.

All these theories are mentioned in Cinemablend’s Was Newt Scamander Actually In The Harry Potter Movies?.