There is so much leeway between 'where horses more owned in the past across social classes and cultures' which is an insanely broad question, and 'What year this did happen in' which is insanely specific and almost arbitrary, because it can be googled and you have the answer instantaneously.

An example of a question in between would be: 'Would you consider the Peace of Westphalia to mark the true beginning of international politics as we know it?' or 'Did mutually assured destruction really prevent the US and the SU from engaging one another?' or, more in line with your question, 'What time period in European history was horse ownership most common, and in what country?' or 'Which culture was most engaged in the glorification of horses as a military or working animal?'. You know, more specific but not too specific. It leaves room for discussion and interpretation. Now all you have are bewildered comments, due to the lack of specificity.

Concerning your question- which, again, lacks specifictiy -horse ownership varied extremely per culture, time period and class. There is really no universal answer.