Benjamin didn't just mourn him; he was freaking out. If he could have you just know he would have torn the van apart with his teeth to get Boxer free.
Not helping is his wild, terrified braying that at times sounds like crying or screaming.
Probably more cruel than both of these is when Boxer is taken away in the van - to the vet's (*cough*) - and though you half know it's coming it doesn't make the scene any easier to read. Not only the moment where the animals realise what is written on the side of the van ("Alferd Simmonds, Horse Slaughterer and Glue Boiler, Willingdon") but also when it's described as they shout out for Boxer to get out of the van and how they hear Boxer try to escape but he's too weak. It doesn't help that it's two horses pulling the van and taking him to die.
It gets worse. The pigs have a feast in Boxer's honor...with the beer that was bought by selling Boxer to the knacker.
Let's be honest here: It's harder to find a moment where you're not tearing up.
The ending. What was worse was that Orwell didn't describe Clover's reaction, so we were left to imagine it...
The death of that poor little sheep.
The chickens. Their big dream of the revolution was that, finally, nobody would take their eggs—their potential unborn children—and eat them or sell them. They wanted to raise their chicks so badly. And then the revolution came, and it seemed like that impossible dream was suddenly going to become a reality. And then Napoleon decided that the farm needed more income...
In the same line, Jessie's puppies. They weren't even sold to support Animal Farm; she couldn't even tell herself she was helping the others or being patriotic. They kidnapped her children and she never knew why or what happened to them.
It may be a mercy that she died before Napoleon could reveal what he did do with them...
In the live-action movie, the scene where she goes to Napoleon and pleads with him to give her puppies back. After Napoleon smirks and insists they're just "educating" them (cutting her off in the middle of crying "But they're my babies...!"), Squealer does his vaguely-threatening "You wouldn't want to disadvantage your own puppies, would you?" She can't argue with it. Her expression is nothing short of heartbreaking.