Snape tells the class that he got the scar on his leg during "an unfortunate encounter with a werewolf." This is later revealed to be a lie, as Snape was actually attacked by Remus Lupin when he was a student at Hogwarts.
While Harry doesn't see the injury, he does hear Snape's explanation and likely thinks of him as a somewhat menacing, secretive, and maybe even a bit dangerous teacher. Harry's early impressions of Snape are mostly negative due to Snape's dislike of Harry and his close association with the "enemy" - the house of Slytherin.