I just have no idea how rigerously the data was reviewed. The 95% decline simply does no compute with

4,500,000 in 1959

and even an increase to

7,000,000 in 1968

largely due to increase in recreational horse population.

https://time.com/archive/6632231/recreation-return-of-the-ho...

So that recreational existence at the leisure of our own machinery seems like an optional future humans can hope for too.

Turns out the chart is about farm horses only as counted by the USDA not including any recreational horses. So this is more about agricultural machinery vs. horses, not passenger cars.

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City horses (the ones replaced by cars and trucks) were nearly extinct by 1930 already.

City horses were formerly almost exclusively bred on farms but because of their practical disappearance such breeding is no longer necessary. They have declined in numbers from 3,500,000 in 1910 to a few hundred thousand in 1930.

https://www2.census.gov/library/publications/decennial/1930/...