Trained or not, it is pretty darn impressive for any 45 lb animal to go after an 1800 lb animal that has literal weapons attached to it.
I have heard it said often that dogs are creatures that have no awareness of their physical size...
Even 4 lb. Chihuahuas can be vicious I am told, and bigger dogs run from them.
I presume is because of the proximity of attacked dog genitals to the teeth of the attacking dog.
Dogs a totally unaware of their own size..............
Here is how I saw it:
Pit Bull runs full speed at massive, peacefully grazing Bison.
Massive Bison looks up, sees the dog just feet away.
Huge shaggy head of bison with gigantic, sharp horns is lowered casually at the last second.
As Pit Bull arrives, Bison delivers a deliberate, aimed blow.
Dog runs off at best possible limping speed.
Massive Bison turns its back on retreating Pit Bull and returns unconcernedly to peaceful grazing.
Nature at work.