The Mountain's men
From A Wiki of Ice and Fire
The Mountain's men refers to the men who ride with Ser Gregor Clegane, the Mountain That Rides. They are a dangerous, unsavory bunch. They are all hard-eyed, hard mouthed men; according to Jaime Lannister they would have to be to ride beside a man like the Mountain.
The best that can be said for Gregor’s men is that they are not quite as vile a bunch as the Brave Companions.  Sandor Clegane is familiar with his brother’s men and refers to them to Arya as Gregor’s "pet rats". Gregor never brings them to court.
As a result of their brutality and continued crimes against the general populace of the Riverlands, the group becomes actively targeted by the Brotherhood Without Banners, and Arya Stark memorizes the names of the most prominent and brutal members as part of her dedicated hitlist.
The Mountain's men
A Game of Thrones
After the Tourney of the Hand, Ser Gregor is furious about the trick that the "Knight o’ Pansies" played on him by riding an in-season mare that distracts his stallion. While returning from the tourney with seven of his men, Gregor broods on his loss. An overflowing river detours the group to a nearby alehouse, where he and his men gang rape the brewer’s daughter Layna there. Chiswyck relates the story of the rape later on at Harrenhal, as Arya, in the guise of a common servant, overhears.
After Catelyn Stark abducts Tyrion Lannister, Ser Gregor and his men strike some of the first blows in what grows into the War of the Five Kings, raiding the Riverlands. Lord Tywin Lannister sends them across the Red Fork under the guise of brigands to ravage the holdfasts of Sherrer, Wendish Town, and Mummer's Ford in the hopes of drawing Tully forces away from Riverrun and the Golden Tooth.
When Lord Eddard Stark dispatches a party of men led by Beric Dondarrion to execute Ser Gregor for his crimes, Tywin Lannister ambushes them at the Mummer's Ford. When Dondarrion and his men attempt to retreat across the Red Ford it is Ser Gregor men who take them in the rear.
Thereafter, the Mountain's men join Lord Tywin's host as it marches through the Riverlands, eventually fighting in the Lannister vanguard on the left flank of the Battle of the Green Fork. After the battle, Lord Tywin withdraws to Harrenhal but leaves the Mountain's men and others to raid in the Riverlands.
A Clash of Kings
As the band is foraging and raiding they capture and question various smallfolk. Amongst the people they capture are Hot Pie, Gendry and, Arya Stark of Winterfell, although they are oblivious to her true identity.  Arya quickly comes to know Gregor’s men and their various personality quirks. Gregor and his men take their surviving prisoners to serve Lord Tywin Lannister at Harrenhal.
Lord Tywin Lannister then sends Ser Gregor and his men to destroy Roose Bolton’s host. During this time, they skirmish several times with the Brotherhood Without Banners, and lose four men to Beric Dondarrion’s night raids before returning to Harrenhal, taking up residence in the Wailing Tower.
It is during this stay at Harrenhal that Arya Stark orders the Faceless Man Jaqen H'ghar to kill Chiswyck after overhearing him regaling the men with the terrible tale of the gang rape of the brewer's daughter. 
During Lord Tywin's attempt to break through back to the Westerlands at the Battle of the Fords, the Mountain's men join up with Lord Tywin's host and attack the crossing at the Stone Mill, which is defended by Ser Edmure Tully's forces. Along with the rest of the Lannister force, the Mountain's men are pushed back and forced to retreat.
A Storm of Swords
Following their failure to cross the Red Fork, Ser Gregor and his men retake Harrenhal (lost during their absence), which Roose Bolton has abandoned to Vargo Hoat. Inside, Gregor's men slaughter the inhabitants and defenders except for the cook who opens the gate, the castle smith, and a girl named Pia whom the Mountain and his men rape repeatedly.
Leaving Polliver as castellan and some men (including the Tickler) to hold Harrenhal, Ser Gregor and the rest of his men follow a force sent by Roose Bolton toward Duskendale. The battle goes against the Northmen when the Mountain's men take them in the rear. The few survivors of the battle are harried in their retreat by Ser Gregor's men. Bolton's rearguard is ravaged by the Mountain and his party when they attempt to cross the Ruby Ford.
After kidnapping Arya Stark, the Hound explains to her that it could be someone worse than him that kidnapped her and that she doesn’t know his brother. Arya protests that she does know Gregor and then names some of Gregor’s men. This surprises Sandor who is amazed that a highborn girl like Arya Stark should know them.
At some point after Polliver is left as castellan, Arya and Sandor Clegane confront some of Gregor’s men who have gone to drink, whore, and "tickle the innkeep" at the Crossroads Inn. A fight breaks out when they attempt to capture Sandor and Sandor and Arya kill Polliver, the Tickler and the Sarsfield boy. 
A Feast for Crows
The remnants of Ser Gregor's men-at-arms, including Raff the Sweetling, Dunsen, and Shitmouth, remain stationed at Harrenhal. When Ser Jaime Lannister arrives there on his way to Riverrun, a handful of Ser Gregor’s men emerge from the castle’s towers to watch him dismount. Ser Jaime discovers that Ser Gregor had named Polliver as his castellan after his departure, but Shitmouth informs him that Polliver was killed in a run-in with Sandor Clegane at the Crossroads Inn, leaving no one in charge at Harrenhal.
Jaime asks them if they sent men after the Hound and the peasant boy he was with. Shitmouth tells him no, as the dog was Ser Gregor's brother. Raff the Sweetling, who Jaime thinks of as bolder than the rest of the remaining men-at-arms, says to Jaime,
|“||We’re bad m’lord… but you’d have to be mad to face the Hound.||”|
Jaime asks Raff if he is afraid of the Hound. Raff answers that he is not so much afraid as leaving him for their betters to take care of, such as Ser Jaime. Jaime thinks to himself that when he had two hands he would have been capable of that task, but he does not delude himself, and knows that Sandor would make short work of him now.
After viewing Vargo Hoat’s head, Jaime asks Shitmouth where the rest of him is. Shitmouth tells Jaime that the rest of him has rotted or been eaten and that Ser Gregor fed his flesh to the captives – referring to Vargo’s flesh as “roast goat”. Jaime thinks to himself that his father’s dogs have gone mad and that somehow revenge has lost its savor.
When Jaime informs Gregor’s men that they can either ride with him to the Siege of Riverrun or join Ser Bonifer Hasty’s men, the men look at one another. One tells Jaime that they’re owed, that Gregor promised rich rewards for those that ride with him.  The men all begin to yammer in agreement. They all assumed rich rewards meant gold. Jaime tells them to take any grievances up with Cersei.
Notably, Dunsen is never encountered personally by Jaime, but confirmed as alive and part of the group in the appendix.
Before he leaves, Jaime takes the head of one of Gregor’s men who tried to rape Pia. As the man is about to be beheaded he protests that he had her hundreds of times, and that they all did.
A Dance with Dragons
Lord Tarly mentions how Gregor's men arrived in Maidenpool while he was in charge of the town. They were not there a day when one was accused of murder and another was accused of rape, Randyll executed the murderer and gelded the rapist.
When they arrived at King's Landing with Ronnet Connington, Lord Randyll suggests they be sent to the Wall where such scum belong. Mace Tyrell agree's and states he would not suffer such men in the City Watch.
References and Notes
- ↑ 1.0 1.1 1.2 A Feast for Crows, Chapter 27, Jaime.
- ↑ 2.0 2.1 A Storm of Swords, Chapter 47, Arya.
- ↑ A Game of Thrones, Chapter 43, Eddard.
- ↑ A Clash of Kings, Chapter 26, Arya.
- ↑ A Clash of Kings, Chapter 30, Arya.
- ↑ A Storm of Swords, Chapter 74, Arya.
- ↑ A Feast for Crows, Appendix.
- ↑ A Feast for Crows, Chapter 27, Jaime
- ↑ A Feast for Crows, Chapter 44, Jaime
- ↑ A Dance with Dragons, Epilogue.