The Rohirrim, why horses?

[Inziladun]

Originally Posted by Morthoron

For instance, the Numenorean infantry was the most formidable force of the 2nd Age. Sauron's legions folded up their tents and headed for the mountains at the very sight of them.

Indeed. The Númenóreans (who effectively saved Eriador twice in the Second Age, when Sauron was too much for the mighty Eldar) weren't much into cavalry.

But [the Númenóreans] did not use [horses] in war....they were of great stature and strength, and their fully-equipped soldiers were accustomed to bear heavy heavy armour and weapons.

UT Disaster of the Gladden Fields (footnote 7)