Horseshoes
After discussing them with my 4-year-old, I fell down a rabbit hole about horseshoes. Are there different types? Like, is there a horseshoe version of, say, sneakers? Turns out horses have many types to choose from, but mostly corrective shoes for various (painful-sounding) hoof maladies.
The funnest fact was that the Romans shod their horses in hipposandals, similar to a modern hoof boot.