How Tsume did get his scar is one of the best kept secrets of the series. (It's one of the things revealed in the OAVS. If you want to know, click here for a spoiler pic. Else, read on.) But one can always speculate. I'm not even sure what kind of scar it is: a flay wound scar, a burn or maybe a local allergic reaction to leather (note how he never zips up the jacket)? Then again, it only looks plausible on the (illusory) human form; in wolf form, it looks like a piece of felt stuck on with glue. One simply cannot have such a neatly delineated scar on thick fur; even if, as I suspect, the skin is so damaged in that spot that no new fur will grow on it, the fur surrounding the scar tissue would lie over it, giving it a very fuzzy outline. A remote possibility is that the fur growing off the scarred skin is itself a different colour - the hair on a horse's healed saddle sore is always a tell-tale white. However, I would be surprised to see a wolf with a saddle sore on its chest.
So, in no particular order, these are my theories:
(Joke from Alas! Smith and Jones:) Fell asleep on the barbie.
Wolves should not use domestic kitchen appliances.
Leg wax incident.
Zorro tried to carve his initial on what he thought was a small boulder.
Wolves should not get a tattoo.
Bumped into an electric fence.
It was supposed to be a heart with an arrow through it and two initials, but the other wolf broke a nail halfway through.
Got grazed by the limbo pole.
A lesson to knock before entering a bear's burrow.
Experimented with passing himself off as cattle instead of human at a place called the "X Ranch".
The neighbours had a really savage cat.
After much tugging, the chewing gum came off - as did most of the hair.
Wolves should not try to shave - not even to allay humans' suspicions over never showing stubble.
Scooped up on a very jagged antler.
Tried to wear real leather jacket and got caught in the zipper.
Knife fight - the other guy's knife was real.
It may look like a scar, but actually - shhh - it's a birthmark.
Had it professionally done to impress the bitches; had no idea it would also attract inept teenage wolf-boys.
Stepped on a button when he was Toboe's age.
Mastiff reacted very emotionally to the taunt of "Why don't you go pick up the newspaper, and fetch my slippers while you're at it".
Wolves should not drive motorized vehicles, especially when they can't reach the brake pedal.
That squirrel put up quite a fight.
Mum tried to eat what she thought was the afterbirth.
Wolves should not play with chemistry sets.
Broke a bottle during bar fight and accidentally stuck jagged end in own chest. Decided to stick with teeth from that day on.
Thinking he'd be safe with native Americans, sought refuge with the Mandan tribe (the ones who suspend themselves from hooks in the chest). Faith in humanity shattered.
Tried to slide down the banister and ended up using his chest as brake.
Ate apple pie at MacDonalds. Most of pie fell on chest. Burn marks on tongue less immediately visible.
And lastly (from Monty Python and the Holy Grail): it was