Soon after being announced in January, doubt was cast over welterweight Horn's dream fight with the 38-year-old Pacquiao as it emerged Khan was his preferred opponent.
The Horn-Pacquiao fight would have been one of the biggest in Australian boxing history.
Meanwhile, one of the few fighters to beat Pacquiao, Floyd Mayweather, has launched a heated attack on UFC superstar Conor McGregor.
The 49-0 retired boxing legend says he is getting sick of McGregor's lip service without much action behind putting together a mooted mega-fight between the pair.
Mayweather accused the UFC lightweight champion of pretending he wants to fight but then failing to sign the contract to make it a reality.
"Conor McGregor, stop blowing smoke up people's a** you little b****," Mayweather said. "If you want to fight, let's make it happen, you punk. Conor McGregor, you don't even want to fight you little b****, you little punk. Let's make it happen."
Mayweather said McGregor's constant chatter about the fight is a common trope he saw throughout his boxing career where competitors would talk a lot about something they supposedly wanted to happen yet really hoped it didn't come together.
"There's always been a lot of talk about Conor McGregor. My thing is this, a lot of times with competitors and athletes and fighters, we can ask for certain things that we really don't want. There's a lot of barking but there's no biting. When you talk about biting, sign the contract. Don't talk the s***. If you really want to fight, sign the f***ing contract and we can make it happen.
"So just letting the world know that he's full of s***. He don't really want to fight. 'Oh I want to fight, I want to fight, I want to fight, Mayweather's scared of me' - well listen, I'm my own boss, let's make the fight happen. You talking all that s***, you acting like you want to fight, I don't think you really want to fight. Sign the contract and let's make it happen."
Mayweather may have strong words for McGregor about making the fight happen but it certainly appears the gap between the two combat sports superstars remains the money they would split on a potential pay-per-view offering.
Mayweather has insisted all along that he would get the bigger share of the money considering his credentials as the biggest draw in the history of boxing.
That said, Irishman McGregor has risen to the top of the pay-per-view ranks while conquering two divisions in the UFC and putting together some of the largest selling cards over his past few fights.