[The Might]

Their connection to horses can be traced back to their ancestors who lived in Rhovanion, having settled there some time before 1250 TA.
Living in an area of large, grassy plains it was useful for them to ride horses to get around quickly and perhaps also to use them for farming.

From them the Éothéod descended, whose name does mean horse-people. They moved north between the Langwell and the Greylin and later rode back south to the aid of Gondor and received Rohan in return.

Again, in Rohan it was useful to have horses since the large, grassy plains made it ideal to ride around.