The vitriol spewing from the US media and sport stars has been swift and damning. The calls of rigging echoed another controversial Pacquiao loss, that one being at the hands of Timothy Bradley jnr five years earlier.
If you believed the hype Horn was beaten beyond reproach. Instead he was given the 117-111, 115-113, 115-113 victory. Based on the scoring system boxing uses, Horn was the clear winner.
No, maybe not 117-111 as judge Waleska Roldan scored it, but 115-113 was about right.
From where I was ringside, Horn ruled the roost for the first six rounds before Pacquiao woke up and battled to get back into the fight.
Pacquiao was ferocious during rounds nine and 10 but the damage had been done - not to Horn's face, but to Pacquiao's scorecard.
Horn won more rounds through the contest. That's what the judges base their decision on. I had it 116-114 to the local.
Both Pacquiao and his iconic promoter, Bob Arun, refused to call the result a robbery. Both astute readers of the sport knew the fight was close, but the Filipino senator left the door wide open.
Horn utilised his height (175cm to 166cm) and reach (172cm to 170cm) to put Pacquiao on the back foot in the early billing. Pacquiao fought back well to almost take the clash in the ninth round before Horn rallied to end the bout on a run.
Both fighters were bloodied, though that was more from stray elbows and head clashes than any blockbluster punch.
Maybe it was the fact the judges couldn't hear the ESPN commentary that heavily favoured Pacquiao. Maybe it was the predominantly Australian crowd, although the Filipino faithful were vociferous. Or maybe it was purely the eye test that swayed the judges and crowd to Horn's side.
There are rumours the rematch between the pair is likely to be Pacquiao's career swansong, though the prospect of a rematch against Floyd Mayweather jnr, arguably the finest pound-for-pound boxer in history, must stoke the fires. This loss, however, may have ended any hope of that.
But for now, let's bask in the glory of the underdog. The fighting schoolteacher who wasn't given a chance is now the world champion. Good on you, Hornet!