The Antler Men are a group of rich traders and merchants of King's Landing. They name themselves after the crowned stag of House Baratheon.
A Clash of Kings
The Antler Men plot betrayal against King Joffrey I Baratheon and intended to abet the fall of the city to Stannis Baratheon. They arm several hundred men and plan, once fighting between Joffrey's loyalists and Stannis's forces has begun, to seize the Old Gate and let the enemy into the city. Master armorer Salloreon is a member of the group.
During the Battle of the Blackwater, captured Antler Men have antlers nailed to their heads and are flung over the walls of King's Landing by the three great trebuchets called the Three Whores.
A Storm of Swords
Tyrion Lannister finds out that some of the Antler Men owed considerable sums of money to the crown.
A Feast for Crows
Jaime Lannister discovers that another of the Antler Men was a cloth merchant who had purchased the position of Chief Gaoler of the Red Keep's dungeons from Petyr Baelish.
Varys: It seems they believe that Lord Stannis must win, and wish to share his victory. They call themselves the Antler Men, after the crowned stag.
Tyrion: Someone should tell them that Stannis changed his sigil. Then they can be the Hot Hearts.
and Tyrion Lannister