Direct copy from the wiki here:
http://enterthegungeon.gamepedia.com/The_BulletRed-Caped Bullet Kin will rarely appear in random rooms after at least one Past has been defeated. These special Bullet Kin wander aimlessly and do not attack - they have guns, but do not use them. If the player stays in a room alone with the special Bullet Kin and does not kill it, it will disappear after a short time. After this occurs five times, The Bullet will be unlocked. Multiple Red-Caped Bullet Kin can be found in a single run, and they are more likely to appear in later chambers.