Capital letters make no difference in how the game finds each folder, just as long as the letters are all there it'll work. Which folders you can use are hard-coded, what's there is there. You cannot create new folders in the Textures\Men\ folder.

You've also got something wrong with the way you've implemented the unit in unit_pod.txt.
FullPlAr is only foot knights (there are NO mounted units). Your new unit is mounted on a horse, yes? Well you'll need a different texture than what you have (change FullPlAr in unit_prod.txt to something else, e.g. MKnight).