Download and save the file on your computer, then extract to a new folder. 

Locate the Stardew Valley game folder on your computer's hard drive, then locate and open the Content folder. 
Always back up your game files before using mods. 
Inside the Content folder of Stardew Valley open the Animals folder and replace horse.xnb with the downloaded horse.xnb file. You will now have an Iridium Bar themed horse!
 
Thanks and good gaming to you!  -Pecuniam Art