"Moleskins" (originally) really are just that. As a teenager (1920s), my father-in-law used to trap moles and send the skins in to be made into trousers. For some reason the skins don't need to be cured. They are very soft and hard wearing so were used by people (e.g. miners) who had to work on their knees for long periods.

Latterly, "moleskin" has come to mean a kind of velvety jeans cloth. Probably because of the texture which is velvety like a mole's skin.

Cheers!
IanC