Was looking through my US collection recently when I noticed a bit of a trend. I guess back in the 1940s and 50s a lot of the US states were celebrating centenaries as territories or states. I found at least eight stamps picturing ox pulled carts representing the pioneers.

Gadsden Purchase, US-Mexico border

Minnesota

Utah

Kansas

Oregon

Swedish pioneers

California

Washington (it's in the seal)
Just thought it was interesting. Obviously oxen were better at pulling ones worldly belongings thousands of miles.