In the "Tale of Two Brothers" short story it's a water buffalo that gives Taka his scar. Sorry. I don't buy that. If a water buffalo gashed his face, he'd have much more than just one little scar to show for it. In fact the outcome, I think, is most likely too graphic to even draw out (at least for me o_O)
It's a popular fan theory that his father Ahadi gave him his scar, but I have also seen theories that it was Mufasa...and I sorta like that theory the best. I'm sure Taka brought it on himself somehow, but Mufasa always seemed to have somewhat of a short fuse; maybe Taka just pushed the wrong buttons one too many times. Plus yknow how moody those teenagers are. "Temper, temper!" XD
You can view that paw as whoever you think did it, if you think it was a lion. If not, there's the water buffalo for good measure.