Unlock Reed's Armored Horse and Macgruder's Cannon Shotgun:
Complete 'Story' mode and ALL side missions getting 100%. When completed Reed's Armoured Horse is at Macgruder mine and the gun is given to you.

Unlock .69 Ferguson Rifle:
Defeat Hollister

Unlock Cavalry Sword:
Defeat Reed

Dual Peacemakers:
Defeat Hoodoo Brown

Unlock Apache Shirt:
Complete ALL Hunting Missions

Silver Spurs:
Complete ALL Pony Express Missions