A comment my mother made, from her experiences as a child in the UK in the 1920s was that if London hadn't moved to mechanical transport, the city would have drowned in horse-shit if the population rise continued. It was a nightly obligation to deal with the prodigious amounts of horse manure, every day, without fail.

I might add that many fine multi-million pound houses now exist inside the horse stables, the "mews" of London. They're a feature of many films in the 50s, 60s and 70s.