What about Ace Bonner's dog...

He appeared in important parts of the movie. When JR meets his mother--the dog growls but Junior walks past him unscathed, and hugs Ida.
He's there when Ace steals Jr's horse. And he's right there in the 'wild milking' event--there's great symbolism--rampaging men trying to get milk from rampaging cattle--something only Peckinpah would zero-in on. And the dog is there again at the start of the barroom brawl, showing his teeth. It could be Peckinpah's teeth.