This is something that's been bothering some people.
In the Lion King:Six new Adventures, it says that Scar's name use to be taka and that he got his Scar from a water buffolo.
For more info on it read The Lion King:Six New Adventures.
My Personal Opinion:I
personally believe that Sarabi had given Taka the scar. I think he had a liking of her and well...it's a popular theory but
I'm sure Sarabi really didn't like his advances. I haven't really thought about much of the details on my theory
but Sarabi giving him the scar is what I believe. I'll post more information once I think of something.