Custom Couriers are cosmetic items that can be equipped in the Courier slot of the loadout screen. Upon being equipped, any purchased couriers will be replaced by the Custom Courier. This change is purely cosmetic. There are 185 custom couriers in Dota 2.
Unusual couriers are found from some Treasures at roughly 1-2% chance. They come with a special effect granted by an Ethereal Gem and a Prismatic Gem. Their effects can be customized by applying different Ethereal and Prismatic gems.