"He's got dog in him"

When did this expression go from meaning a quitter or coward to meaning someone tough and willing to dig deep?

How did it go from one meaning to its opposite?

EDIT: FROM MY POST BELOW (for those too lazy to scroll down):
Just a quick internet search google news archive turned up....
Earnie Shavers before challenging Holmes in 1979: "I really think Larry has a little dog in him. I'm going to put a body attack on Larry and them we'll find out"
James Tillis after going 10 rounds with Tyson in 1986: "I've got some pit bull in me, but I ain't got no dog."
Don King 1990 on Buster Douglas: "Buster Douglas is a dog. He's a quitter, always been a quitter, and always will be a quitter. Really, that's why I chose him"
Eddie McCabe, Canadian sportswriter 1978, looking back on Ali's career: "Clay fought on and Liston eventually quit on his stool, betraying the dog in him which none had suspected behind that malevolent stare."
After KOing Jose Luis Garcia in 1975, Ken Norton said: "I wanted to make the dog in him come out"

Riddick Bowe in 1991 about Bruce Seldon: "'He showed he has some dog in him,' Bowe said of Seldon. 'He got hit a few times and he quit.'
I also remember others, such as Ferdie Pacheco, dismissing Riddick Bowe for having "dog in him", before Bowe proved them wrong.

 

No, trust me, I'm certain it didn't.
A boxer labelled a "dog" was a quitter. A boxer said to have "a bit of dog in him" was one that might quit when it got hard. A boxer who had "no dog in him" was a tough guy who'd never quit.

I'm not even going back that far, 1990s they were still saying it, 2000s even.
It's only in the last few years I notice people using it in absolute opposite sense.

 

Just a quick internet search google news archive turned up....
Earnie Shavers before challenging Holmes in 1979: "I really think Larry has a little dog in him. I'm going to put a body attack on Larry and them we'll find out"
James Tillis after going 10 rounds with Tyson in 1986: "I've got some pit bull in me, but I ain't got no dog."
Don King 1990 on Buster Douglas: "Buster Douglas is a dog. He's a quitter, always been a quitter, and always will be a quitter. Really, that's why I chose him"
Eddie McCabe, Canadian sportswriter 1978, looking back on Ali's career: "Clay fought on and Liston eventually quit on his stool, betraying the dog in him which none had suspected behind that malevolent stare."
After KOing Jose Luis Garcia in 1975, Ken Norton said: "I wanted to make the dog in him come out"

Riddick Bowe in 1991 about Bruce Seldon: "'He showed he has some dog in him,' Bowe said of Seldon. 'He got hit a few times and he quit.'
I also remember others, such as Ferdie Pacheco, dismissing Riddick Bowe for having "dog in him", before Bowe proved them wrong.

 

I think perhaps those old guys used to kick their pet dogs, so they knew dogs were quitters.
Before animal welfare was a thing.

 

Yeah I guess we're getting old. Anything before 2010 is probably prehistoric to most people here, so they have no idea how the meaning of this term did a 180 degree turn. Seriously I doubt it even goes back that far. I mean it might be more like 2016 when it changed.

To be fair, accusing a boxer of having "dog in him" was always a very impolite, nasty and even risky business. I'm sure guys like Pacheco and probably Cosell did it a fair bit, and a ton of sports/boxing writers, never mind other fighters.