I'll name them rather mundane things at first. 1st Hannover foot, 2nd Vienna dragoons, ect. As units grow in experience and distinguish themselves in my army, elevating them above the rank and file they'll be given special names.

I'll rename them based on battlefield performance. Say I have an infantry unit that holds the center of my line in a particularly bloody battle, and doesn't route despite all the losses my forces take. I might rename them the "immovable brigade" or something.