A mod that will change your horse into a  ww2 tank of your choice. So far there is only a Soviet T-34 and a Panzer III.
So riding the tank doesn't look weird you will have to swap out the farmer sprite too. 

BEFORE YOU INSTALL BACK UP ALL THE FILES YOU WILL BE REPLACING.

Tanks included: 

Panzer III

Soviet T-34



To install go the new farmer sprite go to:

C:\Program Files (x86)\Steam\SteamApps\common\Stardew Valley\Content\Characters\Farmer

If your character is female replace "farmer_girl_base.xnb" with the one included in the file you downloaded.
if your character is male replace "farmer_base.xnb" with the one included in the file you downloaded.

To install the actual tank go to:

C:\Program Files (x86)\Steam\SteamApps\common\Stardew Valley\Content\Animals

and replace the "horse.xnb" with whichever tank you prefer. Which is also called "horse"