McClaggan was a British wizard who attended Hogwarts School of Witchcraft and Wizardry in the 1920s. He was a Griffindor student in a Defence Against the Dark Arts Class being taught by Professor Dumbledore in September 1927 when the class was interrupted by Torquil Travers. He defended Dumbledore before being told to leave the class with Professor McGonagall (CG).
McLaggan or McClaggan is a Scottish alternative spelling of the Irish McLaggen.
There has been a non-canon suggestion that this lad might be related to Cormac McLaggen - Gryffindor student and Slug Club member who unsuccessfully competes to become Gryffindor Keeper and takes an unreturned liking for Hermione Granger (HBP).
The first name of McClaggan (CG) is not mentioned and the surnames are spelled slightly differently.