Your hook's form "construction" is what is known as a "doubled brass wire" hook. At first glance, it resembles a civil war era US army knapsack strap-hook. But that type's length is about 1.5-inches, and the ruler shows yours is 2-&-1/8-inches long. The only brass doubled-wire one I know of which is that size is a US Model-1878 bayonet scabbard attachment-hook. However, that hook's "tongue" is not shaped like yours. See the photo below. But note, the hook end on yours is very oddly shaped, looking like it got bent out of its original shape, and was "repaired" by using pliars to bend it back into a useable shape. (I'll say more about that in the paragraph below).
You said you found yours "at an old horse harness race track back in 1890-1920." So, combining all the facts about it, I can think of three possibilities:
1- The Harness-Race (which is a specific type of horseracing) track was built on a late-1800s army training site, where a US Model-1878 bayonet scabbard was lost.
2- As I said above, your hook's end looks bent out of its original curved shape and repaired by re-bending. Also, it's top has been bent out of its original shape. Perhaps it was pried off of a discarded Model-1878 bayonet scabbard and re-used on a horse harness.
3- It may have been manufactured as an early-1900s horse harness strap-hook, and its resemblance to the Model-1878 bayonet scabbard's hook is purely coincidental.
