You rang, Teal? xD
According to the Six New Adventures (a series of official Disney licensed books that were released shortly after the first movie) book, "A Tale of Two Brothers", Taka got his scar after an attempt to embarrass Mufasa backfired and he was gored by the largest Buffalo in Boma's herd. The book can be read
here.
That's the most official version, at least. There's lots of fan-theories out there, the most common being Ahadi gave Taka his scar.